(via ICA Early Warning Flash, No.68, April 5, 1993) GIA GTL writes:
Background
The diamond described herein is a 0.88 carat heat-shaped brilliant that was initially submitted to the GIA Gem Trade Laboratory for grading. During preliminary examination, however, a staff gemologist noted at first appeared to be an extremely low relief fingerprint inclusion containing minute voids. As this would be very atypical for diamond, the stone was brought to the identification and research lab for investigation.
Microscopic features
Examination under magnification using standard darkfield illumination revealed several very transparent, colorless, filled fractures. These all contained minute voids as mentioned above, as well very subtle orange and, to a lesser extent, blue flash effects. Difficulty in detecting these effects was compounded by the very shallow angles of the fractures to the surface of the diamond.
The treatment became more apparent when a pinpoint fiber optic illuminator was used. This lighting technique revealed the extent of the filled breaks, including one very large fracture beneath and nearly parallel to the table. The intense illumination made the flash effects significantly more noticeable, as well as revealing hairline fractures in the filling material. The outlines of the filled areas were also found to be easier to detect when examined in transmitted lighting with a single polarizing filter placed between the microscope’s objectives and the diamond.
Additional testing
Qualitative chemical analysis using energy dispersive X-ray fluorescence detected lead, an element previously detected in diamond fillings, X-radiography further confirmed the presence of the filling in the form of white, X-ray opaque areas on the radiograph.
Discussion
Although the diamond under investigation contained extensive filled fractures, the diagnostic microscopic features of the treatment were quite subtle. These might easily be overlooked if only darkfield illumination were used. It is therefore recommended that additional lighting techniques be used when examining diamonds for possible filled breaks, including pinpoint fiberoptic illumination and polarized light.
Tuesday, April 24, 2007
Diffusion Treated Corundum In Pink To Red To Purple Color Range
(via ICA Early Warning Flash, No.69, May 14, 1993) GIA GTL writes:
Background
The diffusion treated stones described herein were provided for examination by United Radiant Applications, a Southern California-based firm that has been involved in the commercial production of blue diffusion treated sapphires. The faceted specimens, which included 27 stones in the red to pink to purple color range, were made available so that their gemological properties and identification criteria could be documented prior to any commercial release.
Visual appearance
Face-up some of the stones appear uniform in color, while other exhibit uneven color distribution, the latter apparently due to an absence of color on some pavilion facets.
Magnification
A number of features previously documented with blue diffusion treated sapphires were noted in these stones. These include uneven coloration from one facet to another, color concentrations in surface-reaching cavities and fractures, and color reinforcement of facet junctions, although the latter was often significantly more subtle than what we have encountered with blue diffusion-treated stones. Also noted was a type of surface and near surface damage, including minute spherical voids that we had not previously documented in blue diffusion treated sapphires.
Refractive indices
Values were generally higher than those normal for corundum, including some reading over the limits of the conventional refractometer (1.80 +).
Pleochroism
Some stones exhibited atypical dichroism, including a brownish yellow dichroic color.
Short wave UV luminescence
The majority of the stones showed a patchy bluish white luminescence to this wavelength at the surface that was sometimes confined to specific facets or groups of facets.
Absorption spectra
These were generally consistent with those of both natural and synthetic corundums of comparable color, although some absorption features were less pronounced.
Discussion
The diffusion treated corundums described herein are not difficult to identify. Key features include unusually high refractive index readings, atypical dichroism and UV luminescence, patchy surface coloration, color concentrations along facet junctions, and spherical voids just below the surface.
Background
The diffusion treated stones described herein were provided for examination by United Radiant Applications, a Southern California-based firm that has been involved in the commercial production of blue diffusion treated sapphires. The faceted specimens, which included 27 stones in the red to pink to purple color range, were made available so that their gemological properties and identification criteria could be documented prior to any commercial release.
Visual appearance
Face-up some of the stones appear uniform in color, while other exhibit uneven color distribution, the latter apparently due to an absence of color on some pavilion facets.
Magnification
A number of features previously documented with blue diffusion treated sapphires were noted in these stones. These include uneven coloration from one facet to another, color concentrations in surface-reaching cavities and fractures, and color reinforcement of facet junctions, although the latter was often significantly more subtle than what we have encountered with blue diffusion-treated stones. Also noted was a type of surface and near surface damage, including minute spherical voids that we had not previously documented in blue diffusion treated sapphires.
Refractive indices
Values were generally higher than those normal for corundum, including some reading over the limits of the conventional refractometer (1.80 +).
Pleochroism
Some stones exhibited atypical dichroism, including a brownish yellow dichroic color.
Short wave UV luminescence
The majority of the stones showed a patchy bluish white luminescence to this wavelength at the surface that was sometimes confined to specific facets or groups of facets.
Absorption spectra
These were generally consistent with those of both natural and synthetic corundums of comparable color, although some absorption features were less pronounced.
Discussion
The diffusion treated corundums described herein are not difficult to identify. Key features include unusually high refractive index readings, atypical dichroism and UV luminescence, patchy surface coloration, color concentrations along facet junctions, and spherical voids just below the surface.
Mixed Diamonds & Octahedral Cubic Zirconia Baguettes
(via ICA Early Warning Flash, No.73, August 13, 1993) GII writes:
Recently we have encountered in a packet of rough diamond one octahedral shaped cubic zirconia. The person who had done this also must be having some idea of crystallography for he has taken the trouble to etch out trigons on the octahedral faces. Fortunately, the trigons are parallel to the sides of the octahedral faces, whereas in diamonds they are not; this gave us the first doubt and other tests confirmed our suspicion.
Another interesting case of cubic zirconia fraud has also been detected at our laboratory. In a packet of diamond baguettes as well as round brilliant same sized cubic zirconia baguettes and round brilliants were detected. The suspicion was first triggered off when similar dimension baguettes were weighing 0.029 carats and some 0.05. The difference was obvious when the weight of some more number of pieces was compared and other tests were performed.
Recently we have encountered in a packet of rough diamond one octahedral shaped cubic zirconia. The person who had done this also must be having some idea of crystallography for he has taken the trouble to etch out trigons on the octahedral faces. Fortunately, the trigons are parallel to the sides of the octahedral faces, whereas in diamonds they are not; this gave us the first doubt and other tests confirmed our suspicion.
Another interesting case of cubic zirconia fraud has also been detected at our laboratory. In a packet of diamond baguettes as well as round brilliant same sized cubic zirconia baguettes and round brilliants were detected. The suspicion was first triggered off when similar dimension baguettes were weighing 0.029 carats and some 0.05. The difference was obvious when the weight of some more number of pieces was compared and other tests were performed.
Synthetic Diamonds: Rough And Treated, Faceted
(via ICA Early Warning Flash, No.74, August 13, 1993) GIA GTL writes:
Background
Recently a 0.74 carat yellow rough crystal was submitted to the GIA Gem Trade Laboratory in New York for routine identification. Shortly thereafter, a 0.55 carat dark brownish orangy red round brilliant was submitted to GIA GTL for an origin of color determination. Examination of both specimens revealed that they were synthetic diamonds. It was further determined that the rough specimen had been annealed and that the faceted one had been irradiated and subsequently annealed.
Appearance
The habit of the rough crystal was predominantly cubic, with some octahedral and dodecahedral faces. Smooth cube, and to a lesser extent, dodecahedral, faces are seen only on synthetic diamonds.
Magnification/magnetism
Both the rough and faceted pieces contained fairly large inclusions with a metallic luster. When suspended at the end of a thread, both were attached to a magnet, actually attaching to it. This reaction has been noted to date only with synthetic diamonds containing large, magnetic inclusions derived from the metallic flux in which they are produced.
The two showed similar patterns of ultraviolet luminescence and color zoning, forming essentially a square; the faceted piece also revealed this in the form of graining. This growth pattern was centered roughly in the middle of the base of the crystal and on the table of the round brilliant. Octagonal to square patterns are commonly seen in synthetic diamonds, but not in natural diamonds.
Ultraviolet luminescence
The ultraviolet luminescence of both pieces was stronger in short wave than long wave radiation. They both emitted a moderate to strong green fluorescence along the pattern described above in short wave UV radiation, with a weaker green reaction in long wave. The round brilliant also emitted a moderate orange fluorescence in short wave. Gem quality yellow synthetic diamonds previously examined by GIA Research and GIA GTL personnel have shown luminescence in short wave exclusively. The reactions noted in the two specimens under discussion would therefore appear to be from a source other than those we have previously documented in the gemological literature.
UV/Visible/IR spectroscopy
Spectroscopy helped to further characterize the specimens. Infrared spectroscopy showed them both to be essentially type Ib, as are most yellow synthetic diamonds. However, they both also showed a IaA character. This has not previously been observed in gem quality synthetic diamonds.
In the visible range, the round brilliant showed a number of sharp lines between 500 and 700bn at liquid nitrogen temperature. These features were noted both in the hand held spectroscope and on the chart of the spectrometer. Some of these lines have never been observed in natural diamond but have been reported in synthetic diamonds.
Finally, the presence of the features typical of treated pink diamonds in the spectrum of the red brilliant (in addition to other lines mentioned above) and a small HIb peak in the infrared prove that it had been irradiated and subsequently annealed. The yellow crystal also shows a line at about 637nm, which suggests that it, too, had been subjected to annealing.
Conclusion
The above characteristics clearly identify both the crystal and round brilliant as synthetic diamonds. However, some of their properties are slightly different from what we have observed before in yellow gem quality synthetic diamonds. Interestingly, virtually all the features noted are consistent with those of a group of Russian yellow synthetic diamonds currently being studied by GIA Research and the GIA GTL.
Background
Recently a 0.74 carat yellow rough crystal was submitted to the GIA Gem Trade Laboratory in New York for routine identification. Shortly thereafter, a 0.55 carat dark brownish orangy red round brilliant was submitted to GIA GTL for an origin of color determination. Examination of both specimens revealed that they were synthetic diamonds. It was further determined that the rough specimen had been annealed and that the faceted one had been irradiated and subsequently annealed.
Appearance
The habit of the rough crystal was predominantly cubic, with some octahedral and dodecahedral faces. Smooth cube, and to a lesser extent, dodecahedral, faces are seen only on synthetic diamonds.
Magnification/magnetism
Both the rough and faceted pieces contained fairly large inclusions with a metallic luster. When suspended at the end of a thread, both were attached to a magnet, actually attaching to it. This reaction has been noted to date only with synthetic diamonds containing large, magnetic inclusions derived from the metallic flux in which they are produced.
The two showed similar patterns of ultraviolet luminescence and color zoning, forming essentially a square; the faceted piece also revealed this in the form of graining. This growth pattern was centered roughly in the middle of the base of the crystal and on the table of the round brilliant. Octagonal to square patterns are commonly seen in synthetic diamonds, but not in natural diamonds.
Ultraviolet luminescence
The ultraviolet luminescence of both pieces was stronger in short wave than long wave radiation. They both emitted a moderate to strong green fluorescence along the pattern described above in short wave UV radiation, with a weaker green reaction in long wave. The round brilliant also emitted a moderate orange fluorescence in short wave. Gem quality yellow synthetic diamonds previously examined by GIA Research and GIA GTL personnel have shown luminescence in short wave exclusively. The reactions noted in the two specimens under discussion would therefore appear to be from a source other than those we have previously documented in the gemological literature.
UV/Visible/IR spectroscopy
Spectroscopy helped to further characterize the specimens. Infrared spectroscopy showed them both to be essentially type Ib, as are most yellow synthetic diamonds. However, they both also showed a IaA character. This has not previously been observed in gem quality synthetic diamonds.
In the visible range, the round brilliant showed a number of sharp lines between 500 and 700bn at liquid nitrogen temperature. These features were noted both in the hand held spectroscope and on the chart of the spectrometer. Some of these lines have never been observed in natural diamond but have been reported in synthetic diamonds.
Finally, the presence of the features typical of treated pink diamonds in the spectrum of the red brilliant (in addition to other lines mentioned above) and a small HIb peak in the infrared prove that it had been irradiated and subsequently annealed. The yellow crystal also shows a line at about 637nm, which suggests that it, too, had been subjected to annealing.
Conclusion
The above characteristics clearly identify both the crystal and round brilliant as synthetic diamonds. However, some of their properties are slightly different from what we have observed before in yellow gem quality synthetic diamonds. Interestingly, virtually all the features noted are consistent with those of a group of Russian yellow synthetic diamonds currently being studied by GIA Research and the GIA GTL.
Saturday, April 21, 2007
Polymer-impregnated Jadeite
(via ICA Early Warning Flash, No.75, November 23, 1993) GIA GTL writes:
Background
Over the last several years polymer-impregnated jadeite has become prevalent in the jade market. This has given rise to some colors of jadeite being routinely tested for the presence of this treatment.
Recently, the GIA GTL in Santa Monica received for identification a 15 carat purple oval cabochon that we identified as jadeite. Subsequent testing determined the stone to be polymer impregnated. To the best of our knowledge, this is the first report of a jadeite of this color that is polymer-impregnated.
Polymer-impregnated ‘lavender’ jadeite
Gemological properties: Gemological testing revealed an index of refraction and visible absorption spectrum consistent with jadeite jade. Specific gravity was measured by the hydrostatic method and determined to be 3.32, which is slightly lower than the norm for jadeite. This is consistent with previous findings of polymer-impregnated jadeite. The stone was inert to longwave ultraviolet radiation. Magnification did not reveal any evidence of treatment. As is often the case with purple jadeite, the origin of the color could be be determined.
Infrared spectroscopy
The spectrum of this stone reveals intense absorptions around 2900cm¯¹ which are not found in natural jadeite. These additional features are due to the presence of an ‘opticon-like’ polymer. This is not surprising, since this type of polymer is the most commonly employed for jadeite impregnation, according to many reports and our own experience.
Discussion
This finding is particularly significant since none of the polymer-impregnated jadeite (or B jade) we have seen so far was purple in color. They have all been green or mottled green and white, some with applied spots of brown. This means that we will now have to expand our routine testing for polymer impregnation to include purple jadeites as well.
New polymers for jadeite impregnation
We have recently encountered two new types of polymer used for the treatment of jadeite, in addition to the three previously described which are wax, an ‘opticon-like’ polymer, and phthalate-like polymer. Since we do not know yet their exact nature, we will refer to them as polymer 4 and polymer 5.
Polymer 4
The infrared spectrum of polymer 4 is very similar—but not identical—to that of wax. In particular, the major absorption is slightly shifted and of different width than that for wax. Jadeites treated with this product do not ‘sweat’ when tested with the thermal reaction tester, as opposed to those impregnated with wax which do. We measured the SG of one stone showing this kind of impregnation at 3.33.
Polymer 5
The infrared spectrum of polymer 5 shows similar absorption features as polymer 4, plus five more in the range of 2950-3150 cm¯¹. The three jadeites impregnated with this material that we studied are inert in ultraviolet radiation. We could measure the SG on only one of them, and the stone floated in the 3.32 SG liquid (methylene iodide). It is interesting to note that two of these stones displayed ‘sweating’ when tested with T.R.T.
Conclusion
These two new polymers have been seen on a few jadeites submitted for identification, and laboratories involved in B-jade detection should be aware of them. They demonstrate the growing variety of polymers that are being used for jade treatment. One reason for this could be the increasing number of companies involved in this treatment.
Background
Over the last several years polymer-impregnated jadeite has become prevalent in the jade market. This has given rise to some colors of jadeite being routinely tested for the presence of this treatment.
Recently, the GIA GTL in Santa Monica received for identification a 15 carat purple oval cabochon that we identified as jadeite. Subsequent testing determined the stone to be polymer impregnated. To the best of our knowledge, this is the first report of a jadeite of this color that is polymer-impregnated.
Polymer-impregnated ‘lavender’ jadeite
Gemological properties: Gemological testing revealed an index of refraction and visible absorption spectrum consistent with jadeite jade. Specific gravity was measured by the hydrostatic method and determined to be 3.32, which is slightly lower than the norm for jadeite. This is consistent with previous findings of polymer-impregnated jadeite. The stone was inert to longwave ultraviolet radiation. Magnification did not reveal any evidence of treatment. As is often the case with purple jadeite, the origin of the color could be be determined.
Infrared spectroscopy
The spectrum of this stone reveals intense absorptions around 2900cm¯¹ which are not found in natural jadeite. These additional features are due to the presence of an ‘opticon-like’ polymer. This is not surprising, since this type of polymer is the most commonly employed for jadeite impregnation, according to many reports and our own experience.
Discussion
This finding is particularly significant since none of the polymer-impregnated jadeite (or B jade) we have seen so far was purple in color. They have all been green or mottled green and white, some with applied spots of brown. This means that we will now have to expand our routine testing for polymer impregnation to include purple jadeites as well.
New polymers for jadeite impregnation
We have recently encountered two new types of polymer used for the treatment of jadeite, in addition to the three previously described which are wax, an ‘opticon-like’ polymer, and phthalate-like polymer. Since we do not know yet their exact nature, we will refer to them as polymer 4 and polymer 5.
Polymer 4
The infrared spectrum of polymer 4 is very similar—but not identical—to that of wax. In particular, the major absorption is slightly shifted and of different width than that for wax. Jadeites treated with this product do not ‘sweat’ when tested with the thermal reaction tester, as opposed to those impregnated with wax which do. We measured the SG of one stone showing this kind of impregnation at 3.33.
Polymer 5
The infrared spectrum of polymer 5 shows similar absorption features as polymer 4, plus five more in the range of 2950-3150 cm¯¹. The three jadeites impregnated with this material that we studied are inert in ultraviolet radiation. We could measure the SG on only one of them, and the stone floated in the 3.32 SG liquid (methylene iodide). It is interesting to note that two of these stones displayed ‘sweating’ when tested with T.R.T.
Conclusion
These two new polymers have been seen on a few jadeites submitted for identification, and laboratories involved in B-jade detection should be aware of them. They demonstrate the growing variety of polymers that are being used for jade treatment. One reason for this could be the increasing number of companies involved in this treatment.
Yellow And Orange Sapphires
(via ICA Lab Alert, No.1, June 2, 1987) AIGS writes:
Background
In 1981, we at AIGS were asked to identify what was undoubtedly among the first heat treated Sri Lankan yellows to come out of ovens of Bangkok. We were told that the stone was treated by an unknown process (which was later found to be heat), and were asked to determine the color stability. This we proceeded to do by performing our usual fade test. This consisted of exposing the stone to heat and light at 1cm distance of a 150 watt spotlight for up to one hour. We were truly surprised when, after a few minutes exposure, the color had become much darker and more brownish (stones treated with irradiation would fade; some untreated Sri Lankan yellows will fade, but in most the color will not change). This change was temporary only; as the stone cooled to room temperature the color returned to normal.
Since this time we have tested over a thousand Sri Lankan yellow/orange sapphires and have found that all of the heat treated yellow to orange stones react in this way. Thus, a simple test for detection of heat treatment in Sri Lankan yellow/orange sapphires is possible.
The test
Place the stone in close proximetry to a source of mild source of heat, such as an incandescent bulb.
Results
Heat treated Sri Lankan yellow/orange sapphire —The color darkens temporarily, becoming more brownish. The deeper the original color, the greater the change. As the stone cools, the color returns to its original state. Control stones should be used so as to detect even slight changes in color.
Irradiated Sri Lankan yellow/orange sapphire —The color will fade, usually within one hour.
Untreated Sri Lankan yellow/orange sapphire —Generally no change, however some stones may show some fading.
Thai/Australian yellow/orange sapphire, heat treated or untreated—no change has been observed in the color of these stones.
(To: Mr N Horiuchi; Subject: Response to comment on Lab Alert No.1) AIGS writes:
Discussion
Mr N Horiuchi has commented on the test we previously described in Lab Alert No.1 to detect heat treatment in Sri Lankan yellow/orange sapphires. Mr Horiuchi stated that this color also did not fade under the same condition as reported on (by) AIGS.
From the above statement it appears that Mr Horiuchi has not understood the text of Lab Alert No.1. As others may also have misunderstood the text, we will describe the test again below.
Heat treatment in Sri Lankan yellow to orange sapphires may be detected by applying a simple fade test. (Caution: This test only works for Sri Lankan stones). Once a yellow/orange sapphire has been identified as definitely originating from Sri Lanka, its color is tested by applying a simple fade test. The stone in question should be placed on a glass (or other nonflammable) platform within ½ cm of a hot 150 watt (or more) spotlight. The idea is to expose the stone to lots of light and heat. After about 15 minutes exposure (as the stone heats up), the color of a heat treated yellow/orange sapphire will have been found to have become slightly darker and more brownish (the deeper the starting color before the test, the deeper and more brownish the color after heating up). This change is temporary only. As the stone cools its color will fade back to the color before the test was started, not, we repeat, not back to the color before the stone was heat treated (by someone else presumably). We believe that is where Mr Horiuchi misunderstood the original Lab Alert.
Other possible reactions
If the stone has been irradiated (either by nature or by man) the color will fade, usually within one hour’s exposure. In most natural Sri Lankan sapphires, however, the color will show no change. Natural yellow/orange sapphires from other sources and synthetic yellow/orange sapphires also show no change.
To make this test more accurate, control stones of similar color to the stone being tested should be used. Then after the stone in question heats up it can be compared to the color of the control stone. In the case of heat treated yellow/orange Sri Lankan sapphires, the change is not subtle in deeply colored stones, and anyone with normal vision should easily detect it, but the comparison must be made quickly before the stone tested cools down.
Dr Kurt Nassau has informed us that under certain conditions, yellow/orange sapphires may get darker upon exposure to some kinds of visible light. We have absolutely no information on exactly what kinds of stones do this or under what conditions. However he has promised us that the subject will be described in detail soon in an article he has written for Gems & Gemology. We have also written an article on the subjects covered in Lab Alerts Nos. 1 and 2 and submitted it to Gems & Gemology. We don’t know when it will appear (or if it will appear). We have had no reply of any kind, even though we sent it 8 weeks ago.
The subject of color in yellow sapphires is extremely complex. We have no illusions that the above information is the last word on the subject. However, over the past ten years we have tested thousands of pieces of yellow/orange sapphire from all sources and this is what we have found. If anyone else could she additional light on the subject, we would love to hear from them.
Dr K Schmetzer replies:
A. Fe³+ or by Fe³+ and Ti³+: Type 1, originating from Nigeria, Thailand, Australia or
B. By a yellow color center: Type 2, originating from Sri Lanka.
By heat or irradiation heat treatment, yellow stones with similar color centers, i.e with absorption spectrum similar to the spectrum of Type 2, but with different stabilities to light or heat are produced.
C. Irradiation treatment, color center: Type 3
D. Heat treatment, color center: Type 4
According to my experience and knowledge, AIGS describes a test for Type 4 stones, and N Horiuchi is dealing with Type 3 stones. Dr Nassau describes Type 2 stones in Alert No.9, and this type of yellow color center may be connected with natural irradiation. The reason for the higher stability of this naturally irradiated yellow compared to artificially irradiated yellow is an unknown but similar results were found by myself with natural irradiated yellow quartz (citrine) and artificially irradiated yellow quartz.
Background
In 1981, we at AIGS were asked to identify what was undoubtedly among the first heat treated Sri Lankan yellows to come out of ovens of Bangkok. We were told that the stone was treated by an unknown process (which was later found to be heat), and were asked to determine the color stability. This we proceeded to do by performing our usual fade test. This consisted of exposing the stone to heat and light at 1cm distance of a 150 watt spotlight for up to one hour. We were truly surprised when, after a few minutes exposure, the color had become much darker and more brownish (stones treated with irradiation would fade; some untreated Sri Lankan yellows will fade, but in most the color will not change). This change was temporary only; as the stone cooled to room temperature the color returned to normal.
Since this time we have tested over a thousand Sri Lankan yellow/orange sapphires and have found that all of the heat treated yellow to orange stones react in this way. Thus, a simple test for detection of heat treatment in Sri Lankan yellow/orange sapphires is possible.
The test
Place the stone in close proximetry to a source of mild source of heat, such as an incandescent bulb.
Results
Heat treated Sri Lankan yellow/orange sapphire —The color darkens temporarily, becoming more brownish. The deeper the original color, the greater the change. As the stone cools, the color returns to its original state. Control stones should be used so as to detect even slight changes in color.
Irradiated Sri Lankan yellow/orange sapphire —The color will fade, usually within one hour.
Untreated Sri Lankan yellow/orange sapphire —Generally no change, however some stones may show some fading.
Thai/Australian yellow/orange sapphire, heat treated or untreated—no change has been observed in the color of these stones.
(To: Mr N Horiuchi; Subject: Response to comment on Lab Alert No.1) AIGS writes:
Discussion
Mr N Horiuchi has commented on the test we previously described in Lab Alert No.1 to detect heat treatment in Sri Lankan yellow/orange sapphires. Mr Horiuchi stated that this color also did not fade under the same condition as reported on (by) AIGS.
From the above statement it appears that Mr Horiuchi has not understood the text of Lab Alert No.1. As others may also have misunderstood the text, we will describe the test again below.
Heat treatment in Sri Lankan yellow to orange sapphires may be detected by applying a simple fade test. (Caution: This test only works for Sri Lankan stones). Once a yellow/orange sapphire has been identified as definitely originating from Sri Lanka, its color is tested by applying a simple fade test. The stone in question should be placed on a glass (or other nonflammable) platform within ½ cm of a hot 150 watt (or more) spotlight. The idea is to expose the stone to lots of light and heat. After about 15 minutes exposure (as the stone heats up), the color of a heat treated yellow/orange sapphire will have been found to have become slightly darker and more brownish (the deeper the starting color before the test, the deeper and more brownish the color after heating up). This change is temporary only. As the stone cools its color will fade back to the color before the test was started, not, we repeat, not back to the color before the stone was heat treated (by someone else presumably). We believe that is where Mr Horiuchi misunderstood the original Lab Alert.
Other possible reactions
If the stone has been irradiated (either by nature or by man) the color will fade, usually within one hour’s exposure. In most natural Sri Lankan sapphires, however, the color will show no change. Natural yellow/orange sapphires from other sources and synthetic yellow/orange sapphires also show no change.
To make this test more accurate, control stones of similar color to the stone being tested should be used. Then after the stone in question heats up it can be compared to the color of the control stone. In the case of heat treated yellow/orange Sri Lankan sapphires, the change is not subtle in deeply colored stones, and anyone with normal vision should easily detect it, but the comparison must be made quickly before the stone tested cools down.
Dr Kurt Nassau has informed us that under certain conditions, yellow/orange sapphires may get darker upon exposure to some kinds of visible light. We have absolutely no information on exactly what kinds of stones do this or under what conditions. However he has promised us that the subject will be described in detail soon in an article he has written for Gems & Gemology. We have also written an article on the subjects covered in Lab Alerts Nos. 1 and 2 and submitted it to Gems & Gemology. We don’t know when it will appear (or if it will appear). We have had no reply of any kind, even though we sent it 8 weeks ago.
The subject of color in yellow sapphires is extremely complex. We have no illusions that the above information is the last word on the subject. However, over the past ten years we have tested thousands of pieces of yellow/orange sapphire from all sources and this is what we have found. If anyone else could she additional light on the subject, we would love to hear from them.
Dr K Schmetzer replies:
A. Fe³+ or by Fe³+ and Ti³+: Type 1, originating from Nigeria, Thailand, Australia or
B. By a yellow color center: Type 2, originating from Sri Lanka.
By heat or irradiation heat treatment, yellow stones with similar color centers, i.e with absorption spectrum similar to the spectrum of Type 2, but with different stabilities to light or heat are produced.
C. Irradiation treatment, color center: Type 3
D. Heat treatment, color center: Type 4
According to my experience and knowledge, AIGS describes a test for Type 4 stones, and N Horiuchi is dealing with Type 3 stones. Dr Nassau describes Type 2 stones in Alert No.9, and this type of yellow color center may be connected with natural irradiation. The reason for the higher stability of this naturally irradiated yellow compared to artificially irradiated yellow is an unknown but similar results were found by myself with natural irradiated yellow quartz (citrine) and artificially irradiated yellow quartz.
Unusual Composite Ruby
(via ICA Lab Alert, No.2, June 1987) AIGS writes:
During June of 1987, a very unusual composite ruby/synthetic ruby was brought in to the lab of AIGS for testing. It consisted of a piece of Verneuil synthetic ruby to which had been joined at the edge a smaller chunk of natural Burmese ruby. The whole stone was then faceted, concealing the join.
Two features are unusual about this stone. First of all, what appears to be glass has been used to join the two together. Gas bubbles were found in the glass area. Secondly, the edges of both the natural and synthetic areas were irregular. The use of glass to join the two together made this possible, as the glass-filled in the irregular surfaces. The entire stone showed signs of heat treatment, with induced fingerprints present in the synthetic section.
Detection
With the loupe or naked eye this stone could fool many people as the join looks like a crack and the natural area contained a large cloud of silk. However, immersion or overhead lighting will reveal the different luster of the glass join in the microscope. In addition, the synthetic portion displays curved striae and gas bubbles, as well as the induced fingerprints. The stone was purposely cut ‘native’ to imitate the appearance of a ruby just brought out from Burma.
During June of 1987, a very unusual composite ruby/synthetic ruby was brought in to the lab of AIGS for testing. It consisted of a piece of Verneuil synthetic ruby to which had been joined at the edge a smaller chunk of natural Burmese ruby. The whole stone was then faceted, concealing the join.
Two features are unusual about this stone. First of all, what appears to be glass has been used to join the two together. Gas bubbles were found in the glass area. Secondly, the edges of both the natural and synthetic areas were irregular. The use of glass to join the two together made this possible, as the glass-filled in the irregular surfaces. The entire stone showed signs of heat treatment, with induced fingerprints present in the synthetic section.
Detection
With the loupe or naked eye this stone could fool many people as the join looks like a crack and the natural area contained a large cloud of silk. However, immersion or overhead lighting will reveal the different luster of the glass join in the microscope. In addition, the synthetic portion displays curved striae and gas bubbles, as well as the induced fingerprints. The stone was purposely cut ‘native’ to imitate the appearance of a ruby just brought out from Burma.
Plastic Coating Of Gemstones
(via ICA Lab Alert, No.3, 1987) AIGS writes:
Details
In the past 3 years, gemologists at AIGS in Bangkok have encountered an unusual type of assembled stone in which an inferior specimen is coated with colored plastic. All of the stones treated in this manner have been of Burmese origin and so it is believed that the treatment is probably done in Burma.
One type consists of a poor color jadeite cabochon coated with a thin layer of rich green plastic. The coating covers all surfaces of the cabochon except the bottom. After coating, the stone appears to be of very high quality.
Another type is a light color faceted ruby coated with red plastic and then repolished. This gives the appearance of a fine ruby.
The third type seen is a white star sapphire cabochon entirely coated with red plastic. The gem then appears like a beautiful star ruby.
Detection
Although extremely deceptive to the naked eye, these plastic coated stones are readily identified under magnification. They may be dangerous to the trade, though, because their appearance is so natural that unsuspecting dealer might not even check them with the loupe. One unaided clue is provided by the slightly warm and plastic-like feel of the stones. This, however, is very subtle.
Identification of plastic coating is made with the microscope. In the case of the jadeite, as the plastic does not cover the stone entirely, it may be seen to peel away from the stone in places along the girdle. In all types, gas bubbles may be visible in the plastic coating, particularly in the star ruby type, where the coating was thicker. Color swirls could also be seen in the star ruby type. Judicious use of the hot point will, of course, also reveal this fraud.
Details
In the past 3 years, gemologists at AIGS in Bangkok have encountered an unusual type of assembled stone in which an inferior specimen is coated with colored plastic. All of the stones treated in this manner have been of Burmese origin and so it is believed that the treatment is probably done in Burma.
One type consists of a poor color jadeite cabochon coated with a thin layer of rich green plastic. The coating covers all surfaces of the cabochon except the bottom. After coating, the stone appears to be of very high quality.
Another type is a light color faceted ruby coated with red plastic and then repolished. This gives the appearance of a fine ruby.
The third type seen is a white star sapphire cabochon entirely coated with red plastic. The gem then appears like a beautiful star ruby.
Detection
Although extremely deceptive to the naked eye, these plastic coated stones are readily identified under magnification. They may be dangerous to the trade, though, because their appearance is so natural that unsuspecting dealer might not even check them with the loupe. One unaided clue is provided by the slightly warm and plastic-like feel of the stones. This, however, is very subtle.
Identification of plastic coating is made with the microscope. In the case of the jadeite, as the plastic does not cover the stone entirely, it may be seen to peel away from the stone in places along the girdle. In all types, gas bubbles may be visible in the plastic coating, particularly in the star ruby type, where the coating was thicker. Color swirls could also be seen in the star ruby type. Judicious use of the hot point will, of course, also reveal this fraud.
Glass Infilling Of Cracks In Ruby
(via ICA Lab Alert, No. 4, June, 1987) AIGS writes:
Details
During the ICA Congress held recently in Bangkok, Dr Henri Hanni of Switzerland described to us a new ruby treatment. This consisted of poor quality African ruby cabochons whose cracks had been filled with glass. At the time of the Congress we had not yet seen these stones in Bangkok. In late June of 1987 we saw the first stone. It was a heavily included ruby cabochon, with many cracks that passed deep into the stone. These were filled with glass-like substance. This treatment differs from ordinary surface repaired rubies as the glass dos not just fill in surface pits, but instead appears to penetrate deep into the cracks.
Detection
This treatment is easily detected in the same manner as ordinary surface repaired rubies. Using overhead lighting, or immersion in methylene iodide, will reveal the glass filling due to its different luster or relief. If the opening of the crack is very narrow, however, the glass filling may be difficult to see. Gas bubbles may be found in some of the glass areas.
Dr K Schmetzer writes:
Kenyan rubies are also treated with plastics in order to improve the quality of the stones. In the treatment, cracks or fissures were filled with plastics which is sometimes deeply penetrating into the stones.
Details
During the ICA Congress held recently in Bangkok, Dr Henri Hanni of Switzerland described to us a new ruby treatment. This consisted of poor quality African ruby cabochons whose cracks had been filled with glass. At the time of the Congress we had not yet seen these stones in Bangkok. In late June of 1987 we saw the first stone. It was a heavily included ruby cabochon, with many cracks that passed deep into the stone. These were filled with glass-like substance. This treatment differs from ordinary surface repaired rubies as the glass dos not just fill in surface pits, but instead appears to penetrate deep into the cracks.
Detection
This treatment is easily detected in the same manner as ordinary surface repaired rubies. Using overhead lighting, or immersion in methylene iodide, will reveal the glass filling due to its different luster or relief. If the opening of the crack is very narrow, however, the glass filling may be difficult to see. Gas bubbles may be found in some of the glass areas.
Dr K Schmetzer writes:
Kenyan rubies are also treated with plastics in order to improve the quality of the stones. In the treatment, cracks or fissures were filled with plastics which is sometimes deeply penetrating into the stones.
Friday, April 20, 2007
Natural And Synthetic Yellow/Orange Sapphires
(via ICA Lab Alert, No. 5, December 1987) AIGS writes:
Subject
The detection of color banding/growth zoning in natural and synthetic yellow/orange sapphires.
Method
Color banding, either straight or curved, can be detected much more easily by using a technique developed at AIGS in 1981. This involves the use of a frosted (diffused) blue filter over the microscope’s light source.
When looking for color zoning in yellow sapphires, the usual practice is to immerse the stone in methylene iodide. However, with a yellow stone in a yellow liquid over a yellow (incandescent) light, there is little chance of finding yellow bands of color. Using a white (fluorescent) light helps a bit, but not enough. AIGS have found that by using a frosted blue filter it becomes a much more easier to locate color bands, either straight or curved, as blue is the color being absorbed the most in yellow stones. Sometimes we stack two or three blue filters on top of one another. Although this does not cut down on the light intensity, it still makes it much easier to locate the color zoning. Using the frosted blue filter plus immersion, it is possible to locate straight or curved color banding in about 95% or more of all natural and synthetic yellow/orange sapphires. Furthermore, a green filter can be used for rubies—the color of the should approximate the absorption maxima of the stone.
E Gubelin writes:
To use a frosted blue filter and examine the gem in immersion is an excellent suggestion (though known to experienced gemologist for many years already). The effect may be enhanced if one close the diaphragm to about half or about a quarter of its diameter below the immersion cell.
Subject
The detection of color banding/growth zoning in natural and synthetic yellow/orange sapphires.
Method
Color banding, either straight or curved, can be detected much more easily by using a technique developed at AIGS in 1981. This involves the use of a frosted (diffused) blue filter over the microscope’s light source.
When looking for color zoning in yellow sapphires, the usual practice is to immerse the stone in methylene iodide. However, with a yellow stone in a yellow liquid over a yellow (incandescent) light, there is little chance of finding yellow bands of color. Using a white (fluorescent) light helps a bit, but not enough. AIGS have found that by using a frosted blue filter it becomes a much more easier to locate color bands, either straight or curved, as blue is the color being absorbed the most in yellow stones. Sometimes we stack two or three blue filters on top of one another. Although this does not cut down on the light intensity, it still makes it much easier to locate the color zoning. Using the frosted blue filter plus immersion, it is possible to locate straight or curved color banding in about 95% or more of all natural and synthetic yellow/orange sapphires. Furthermore, a green filter can be used for rubies—the color of the should approximate the absorption maxima of the stone.
E Gubelin writes:
To use a frosted blue filter and examine the gem in immersion is an excellent suggestion (though known to experienced gemologist for many years already). The effect may be enhanced if one close the diaphragm to about half or about a quarter of its diameter below the immersion cell.
Plastic Treated Emeralds
(via ICA Lab Alert No. 6, August 13, 1987) Nubio Horiuchi writes:
Source
I have personally seen this treatment for the past three years in Japan (Central Gem Laboratory).
Status
Details of this have not been announced yet, but it is summarized as follows:
The fractures are first cleaned and then impregnated with some kind of liquid plastic. It is presumed that the liquid plastic is hardened by irradiation of light or ultraviolet rays.
Merit of this treatment
In normal oil treatment the oil will seep out during cleaning or over a period of normal wearing and there is a gradual loss of color and the fractures become noticeable. However, with the impregnation of liquid plastic the treatment is durable.It would appear from the durability standpoint that the liquid plastic treatment is better than oiling.
Identification
It is difficult to distinguish between oil and plastic treatments.
Question
In which category of enhancement and treatment should the plastic treated emeralds be classified?
E. Gubelin writes:
Though more durable the result of this new plastic treatment should become to known to all members immediately, because many members of the trade use an ultrasonic cleaning machine which causes the oil to be washed out. If no oil is being washed out, people might not become aware of the fact that the fractures are filled with plastic films. Despite the greater durability the stimulus for easier fraudulent practices does by no ways raise the ethical standard of this plastic treatment.
Dr K Schmetzer writes:
The stones should be classified as plastic-impregnated emeralds.
Source
I have personally seen this treatment for the past three years in Japan (Central Gem Laboratory).
Status
Details of this have not been announced yet, but it is summarized as follows:
The fractures are first cleaned and then impregnated with some kind of liquid plastic. It is presumed that the liquid plastic is hardened by irradiation of light or ultraviolet rays.
Merit of this treatment
In normal oil treatment the oil will seep out during cleaning or over a period of normal wearing and there is a gradual loss of color and the fractures become noticeable. However, with the impregnation of liquid plastic the treatment is durable.It would appear from the durability standpoint that the liquid plastic treatment is better than oiling.
Identification
It is difficult to distinguish between oil and plastic treatments.
Question
In which category of enhancement and treatment should the plastic treated emeralds be classified?
E. Gubelin writes:
Though more durable the result of this new plastic treatment should become to known to all members immediately, because many members of the trade use an ultrasonic cleaning machine which causes the oil to be washed out. If no oil is being washed out, people might not become aware of the fact that the fractures are filled with plastic films. Despite the greater durability the stimulus for easier fraudulent practices does by no ways raise the ethical standard of this plastic treatment.
Dr K Schmetzer writes:
The stones should be classified as plastic-impregnated emeralds.
New Treatment For Diamonds
(via ICA Lab Alert No.7, August 13, 1987) Nubo Horiuchi writes:
Source
I found this treatment in January 1987.
Status
This treatment makes cleavage cracks to the surface less visible by impregnating with unknown material. On looking through the cleavage crack of a diamond treated in this manner, a whitish appearance can be seen which improves the clarity grade of the diamond. Impregnating the cleavage crack of the diamond with this unknown material, which may be silicon oil, it is quite effective in improving the appearance of the cleavage crack because it reduces diffuse reflections. This treatment was located in the diamonds lots imported from Israel.
Identification
Upon looking through a diamond under a diamond light, a dark blue or rainbow hue of interference color will be seen under the diffused light.
Opinion
The organization of gem laboratories in Japan judges that diamonds enhanced in this manner are treated diamonds.
E.Gubelin writes:
It certainly is imperative that all members are informed about this new unethical treatment of diamonds because too many dealers might consider these artificially filled fractures as naturally lined fissures.
Youichi Horikawa writes:
I think identification of these treated diamonds is not easy, because the interference color can be seen in untreated diamonds also.
Source
I found this treatment in January 1987.
Status
This treatment makes cleavage cracks to the surface less visible by impregnating with unknown material. On looking through the cleavage crack of a diamond treated in this manner, a whitish appearance can be seen which improves the clarity grade of the diamond. Impregnating the cleavage crack of the diamond with this unknown material, which may be silicon oil, it is quite effective in improving the appearance of the cleavage crack because it reduces diffuse reflections. This treatment was located in the diamonds lots imported from Israel.
Identification
Upon looking through a diamond under a diamond light, a dark blue or rainbow hue of interference color will be seen under the diffused light.
Opinion
The organization of gem laboratories in Japan judges that diamonds enhanced in this manner are treated diamonds.
E.Gubelin writes:
It certainly is imperative that all members are informed about this new unethical treatment of diamonds because too many dealers might consider these artificially filled fractures as naturally lined fissures.
Youichi Horikawa writes:
I think identification of these treated diamonds is not easy, because the interference color can be seen in untreated diamonds also.
New Diamond Treatment
(via ICA Lab Alert No.8, August 14, 1987) GIA GTL writes:
The New York GIA GTL recently examined a group of diamonds which had undergone a ‘fill’ treatment to improve their appearance.
‘We were told the diamonds had been treated in Israel and that this process has been in use for some time,” said Bert Krashes. “In view of the obligation of the jeweler to disclose treatments, this procedure will be yet another challenge in terminology and explanation to the retail customer.”
Apparently, the treatment has been applied only to highly imperfect diamonds with flaws that open to the surface. By introducing a high refractive index fill into fractures and gletzes, they become dramatically less noticeable to the unaided eye. I2 and I3 grades, for example, are improved to an I1 appearance.
Under binocular magnification, the appearance of these cracks is different from untreated ones, showing white thread-like and pinpoint deposits similar to ‘fingerprint’ inclusions. In addition, an orangy brown reflection was observed in the surfaces reached by the cracks. This suggests the color of the filler used may be brownish, typical of high refractive liquids. It has been reported that the filler can be removed by soaking in aqua regia. The treatment is said to now be available in Antwerp as well as Israel.
“The examination was necessarily hurried and only a few diamonds were available to us; therefore this should be considered an alert rather than a definitive description,” said Krashes. “GIA is attempting to secure more of these diamonds for study and will issue a full report as soon as possible.”
The New York GIA GTL recently examined a group of diamonds which had undergone a ‘fill’ treatment to improve their appearance.
‘We were told the diamonds had been treated in Israel and that this process has been in use for some time,” said Bert Krashes. “In view of the obligation of the jeweler to disclose treatments, this procedure will be yet another challenge in terminology and explanation to the retail customer.”
Apparently, the treatment has been applied only to highly imperfect diamonds with flaws that open to the surface. By introducing a high refractive index fill into fractures and gletzes, they become dramatically less noticeable to the unaided eye. I2 and I3 grades, for example, are improved to an I1 appearance.
Under binocular magnification, the appearance of these cracks is different from untreated ones, showing white thread-like and pinpoint deposits similar to ‘fingerprint’ inclusions. In addition, an orangy brown reflection was observed in the surfaces reached by the cracks. This suggests the color of the filler used may be brownish, typical of high refractive liquids. It has been reported that the filler can be removed by soaking in aqua regia. The treatment is said to now be available in Antwerp as well as Israel.
“The examination was necessarily hurried and only a few diamonds were available to us; therefore this should be considered an alert rather than a definitive description,” said Krashes. “GIA is attempting to secure more of these diamonds for study and will issue a full report as soon as possible.”
Yellow Sapphire
(via ICA Lab Alert, No.9, September 1, 1987) Kurt Nassau writes:
Background
There are several types of natural yellow sapphires that are seen in the trade, including the untreated, the high temperature heated, and the irradiated ones. The first two are stable to light, while the third (irradiated either by nature or by man) fades in light. Natural yellow stones after being mined may fade on light exposure, and it is customary to expose such material to light or heat it. A heating test is also sometimes used to check yellow sapphire for fading: Webster recommends 230°C (446°F) for a few minutes and Nassau has used 200°C for one hour to establish a potential for fading in light in irradiated gemstones in general.
Observation
Ordinary yellow sapphire, that is the non-irradiated, non-heated, non-light fading, stable material can lose some color at as low as 60°C (140°F), more at higher temperatures, and all color by 600°C (1112°F). Quite unexpectedly, light has been found to reverse this change. It restores this type of yellow sapphire to its ‘proper’ stable color from either the dark irradiated state or from the lighter heated state. If heating has been performed accidentally, the color may be restored by exposure to bright light for a few days.
Recommendations
Do not use a heating test for any yellow sapphire. To test for irradiated stones, a light exposure test is the only one that can safely be recommended.
Reference
A fully detailed article by Kurt Nassau and G Kay Valente has been submitted for publication in Gems & Gemology under the title “The Seven Types of Yellow Sapphire and a Corundum Conurundum.”
Background
There are several types of natural yellow sapphires that are seen in the trade, including the untreated, the high temperature heated, and the irradiated ones. The first two are stable to light, while the third (irradiated either by nature or by man) fades in light. Natural yellow stones after being mined may fade on light exposure, and it is customary to expose such material to light or heat it. A heating test is also sometimes used to check yellow sapphire for fading: Webster recommends 230°C (446°F) for a few minutes and Nassau has used 200°C for one hour to establish a potential for fading in light in irradiated gemstones in general.
Observation
Ordinary yellow sapphire, that is the non-irradiated, non-heated, non-light fading, stable material can lose some color at as low as 60°C (140°F), more at higher temperatures, and all color by 600°C (1112°F). Quite unexpectedly, light has been found to reverse this change. It restores this type of yellow sapphire to its ‘proper’ stable color from either the dark irradiated state or from the lighter heated state. If heating has been performed accidentally, the color may be restored by exposure to bright light for a few days.
Recommendations
Do not use a heating test for any yellow sapphire. To test for irradiated stones, a light exposure test is the only one that can safely be recommended.
Reference
A fully detailed article by Kurt Nassau and G Kay Valente has been submitted for publication in Gems & Gemology under the title “The Seven Types of Yellow Sapphire and a Corundum Conurundum.”
Diamond Films
(via ICA Lab Alert, No.10, September 1, 1987) Kurt Nassau writes:
Status
Much publicity both in the general press as well as in the trade has recently been focused on thin diamond films grown by a variety of low pressure techniques. Most of this publicity is highly exaggerated.
The facts are the following:
- Thin films of single crystal diamond can be grown on diamond, but the growth rate is so extremely slow that this is of no significance to the gemstone field.
- Thin films of polycrystalline diamond, composed of many tiny crystals, can be grown quite rapidly on a variety of surfaces, but adhesion is mostly very poor. Since such films are not single crystal, their presence should be easier to detect than most other coatings on gemstones. They could only fool a thermal diamond tester if the coating is very thick, when their presence should be immediately evident under the loupe.
Reference
More detail is given in an article being published in Jeweler’s Circular Keystone under the title “New Synthetics: Cause for Panic—or for the Blahs?”
Status
Much publicity both in the general press as well as in the trade has recently been focused on thin diamond films grown by a variety of low pressure techniques. Most of this publicity is highly exaggerated.
The facts are the following:
- Thin films of single crystal diamond can be grown on diamond, but the growth rate is so extremely slow that this is of no significance to the gemstone field.
- Thin films of polycrystalline diamond, composed of many tiny crystals, can be grown quite rapidly on a variety of surfaces, but adhesion is mostly very poor. Since such films are not single crystal, their presence should be easier to detect than most other coatings on gemstones. They could only fool a thermal diamond tester if the coating is very thick, when their presence should be immediately evident under the loupe.
Reference
More detail is given in an article being published in Jeweler’s Circular Keystone under the title “New Synthetics: Cause for Panic—or for the Blahs?”
New Synthetic Alexandrite
(via ICA Lab Alert, No.11, September 1, 1987) Kurt Nassau writes:
Background
In 1976 U.S patent 3,997,853 by R C Morris and C F Cline was assigned to Allied Chemical Corp. This described the Czochralski growth pulling from the melt of alexandrite (chrysoberyl containing chromium and showing a color change). The patent discussed the laser use of such crystals containing only low levels of chromium that produce only a weak alexandrite effect. This matter was covered in my book “Gems Made by Man”.
Status
Higher concentrations of chromium, that give a very good alexandrite effect in very clean material, is now being grown by the Czochralski technique of pulling from the melt by Allied and is about to be marketed by M S B Industries Inc of Hillside, New Jersey. Gemological examination by the author and the GIA is under way and the characteristics of Czochralski grown synthetic alexandrite will be published when the work has been completed. This material can be expected to appear in the trade soon.
Background
In 1976 U.S patent 3,997,853 by R C Morris and C F Cline was assigned to Allied Chemical Corp. This described the Czochralski growth pulling from the melt of alexandrite (chrysoberyl containing chromium and showing a color change). The patent discussed the laser use of such crystals containing only low levels of chromium that produce only a weak alexandrite effect. This matter was covered in my book “Gems Made by Man”.
Status
Higher concentrations of chromium, that give a very good alexandrite effect in very clean material, is now being grown by the Czochralski technique of pulling from the melt by Allied and is about to be marketed by M S B Industries Inc of Hillside, New Jersey. Gemological examination by the author and the GIA is under way and the characteristics of Czochralski grown synthetic alexandrite will be published when the work has been completed. This material can be expected to appear in the trade soon.
Reappearance Of Surface Diffusion Treated Blue Sapphires in Bangkok
(via ICA Lab Alert, No.12, October 21, 1987) AIGS writes:
Discussion
Most gemologists are well aware of the surface diffusion treatment for corundums. A light colored stone is packed in a slurry of coloring agents and heated to near the melting point. This drives the coloring agents just beneath the surface, creating a deeply colored skin. The stone is then lightly repolished.
In the early 1980s many such stones were seen in Bangkok, usually of a blue color, and some stone burners performed the treatment locally. However it seemed that after unscrupulous dealers learned that gemologists could easily spot the treatment, the incentive was gone and such stones largely disappeared from the market. At least they did until this week. Then several stones came to AIGS for testing which had been treated in this way. They differed from those seen in the past in that they were not near colorless stones coated to a deep blue. By skillful repolishing most of the coating was removed, leaving just enough to improve the stone a bit. In other words, the coating made a $100/ct, sapphire into a $150/ct stone. In the past week we have seen over five of these stones. We suspect that one or more local burners may be performing the treatment.
Identification
These stones are much more difficult to identify because most of the surface coating has been removed by careful repolishing, and because the stones do contain a fair amount of naturally occurring color banding inside the stone. Identification may be made, however, by examining the girdle region and facet junctions very carefully under immersion cell. Interestingly enough, horizontal microscopes do not work as well as the Gem Instrument’s ‘Gemolite’ because the light on horizontal microscopes comes from one side only, while in Gemolite the light comes from all directions due to a circular glass diffusing ring. Identifying these diffusion treated stones is made best by careful examination of the stone’s surface under immersion. Color will be found in places to follow the facets, which is impossible in an untreated stone. Again, confusion is easy however, because these stones may contain considerable natural color within the stone. Side-by-side comparison with a known natural stone and a known surface diffusion treated stone will aid in making difficult separations.
Discussion
Most gemologists are well aware of the surface diffusion treatment for corundums. A light colored stone is packed in a slurry of coloring agents and heated to near the melting point. This drives the coloring agents just beneath the surface, creating a deeply colored skin. The stone is then lightly repolished.
In the early 1980s many such stones were seen in Bangkok, usually of a blue color, and some stone burners performed the treatment locally. However it seemed that after unscrupulous dealers learned that gemologists could easily spot the treatment, the incentive was gone and such stones largely disappeared from the market. At least they did until this week. Then several stones came to AIGS for testing which had been treated in this way. They differed from those seen in the past in that they were not near colorless stones coated to a deep blue. By skillful repolishing most of the coating was removed, leaving just enough to improve the stone a bit. In other words, the coating made a $100/ct, sapphire into a $150/ct stone. In the past week we have seen over five of these stones. We suspect that one or more local burners may be performing the treatment.
Identification
These stones are much more difficult to identify because most of the surface coating has been removed by careful repolishing, and because the stones do contain a fair amount of naturally occurring color banding inside the stone. Identification may be made, however, by examining the girdle region and facet junctions very carefully under immersion cell. Interestingly enough, horizontal microscopes do not work as well as the Gem Instrument’s ‘Gemolite’ because the light on horizontal microscopes comes from one side only, while in Gemolite the light comes from all directions due to a circular glass diffusing ring. Identifying these diffusion treated stones is made best by careful examination of the stone’s surface under immersion. Color will be found in places to follow the facets, which is impossible in an untreated stone. Again, confusion is easy however, because these stones may contain considerable natural color within the stone. Side-by-side comparison with a known natural stone and a known surface diffusion treated stone will aid in making difficult separations.
Characteristics Of Heat Treated And Diffusion Treated Corundums
(1982) Henry A Hanni writes:
For the past two years blue sapphires with good color and pleasing size have become more abundant in the trade. Surprisingly, these stones are offered in lots in which the individual stones are uniformly colored. This is not common with larger gems of high value. Initially, we dealt with such sapphires very carefully and objectively due to a vague suspicion.
During the course of our investigation (Bosshart 1981) the subject of heat treatment of corundum has been brought up at various international meetings of gemologists. Finally, Nassau, Crowningshield and Liddicoat went public with some presentations, which if not quite complete are fairly comprehensive. This report is supplementary state of the author to a recent publication (Hanni 1982).
Heat treatment of corundum has long been practiced, on the one hand after indigenous methods in the countries of origin (i.e. Sri Lanka), on the other hand after an industrial method employed since 1973. With local methods, controlling of temperature and environment is not satisfactorily possible. Because of this, the results are unreliable. Application of the industrial method, carried out in the USA and Thailand, guarantees constant, controlled conditions and leads to the desired results. Let us consider the patents of Union Carbide in 1973 as a starting point. In these patents, techniques are described which allow (at first applied to synthetics only) a homogenization or improvement of the color or the development of asterism. Over the last years, these processes have been tried and improved on natural corundum. Presently, masters of such techniques reside in Thailand. Their latest surprising creations are yellow to orange corundums produced by heat treating raw material corundum of an as yet undescribed type.
The aim of heat treatment is to reach a uniform distribution of color, to clarify or to increase the color, or to attain better purity by dissolving certain inclusions. Critical is whether these stones have absorbed foreign matter during the process of heat treatment, or the required improvement was achieved with those components already present in the stone. Recently, Dr Nassau has reviewed the different processes (Nassau 1981, tab 1). These nine types of treatment may be divided into two groups: With Nos. 1-6, a simple heating occurs; with Nos. 7-9, color producing chemicals are added during the process. Case No.6 may be regarded separately, since with this treatment, natural-looking fingerprint inclusions are generated in synthetic stones.
Heat treatment
This represents a normal annealing (but an extremely high temperature), a process which is well-known and applied to amethyst, zircon, topaz, tourmaline, etc. The new color is considered to be stable. Corundum treated after the processes Nos. 1-5 may be traded without complementary designation according to the regulations of CIBJO colored stones commission (CIBJO 1981).
For example, process No. 3 can be demonstrated with blue sapphire as follows: iron and titanium in the form of Fe/Ti-pairs in the corundum crystal lattice are responsible for the blue color in sapphires (Schmetzer & Bank, 1981). If one of the two partners is less abundant, its deficiency cannot be compensated by addition of the other partner. The solubility of iron and titanium is higher at high temperatures than at low temperatures. If a corundum saturated in Fe and Ti cools slowly, TiO2 is able to unmix from the corundum lattice as rutile (or silk). In this way, Ti is eliminated as a color producing component. On the other hand, by reductive heating a light blue rutile containing corundum to 1600°C and cooling it rapidly, one may obtain stones with intense blue color and a better transparency. The rutile has dissolved and joined with the possibly present iron to form the color yielding Fe/Ti pairs (Fe²+ / Ti4+ charge transfer).
Heat treatment processes used on sapphires and rubies (after K Nassau, 1981)
1. Treatment: Heating only
Specific process: Moderate temperature (1300°C)
Result: Develops potential asterism
2. Treatment: Heating only
Specific process: High temperature (1600°C) / rapid cooling
Result: Removes silk and asterism
3. Treatment: Heating only
Specific process: Reductive heating (1600°C)
Result: Develops potential blue color
4. Treatment: Heating only
Specific process: Oxidative heating (1600°C)
Result: Diminishes blue color
5. Treatment: Heating only
Specific process: Extended heating (1800°C)
Result: Diminishes Verneuil banding and strain
6. Treatment: Heating under unknown conditions (this process is used on synthetic material to generate inclusions with a natural appearance)
Specific process: ?
Result: Introduces fingerprint inclusions
7. Treatment: Diffusion of impurities into the material (extended heating at 1800°C)
Specific process: Add TiO2
Result: Produces asterism
8. Treatment: Diffusion of impurities into the material (extended heating at 1800°C)
Specific process: Add TiO2 and / or Fe2 O3
Result: Produces blue color
9. Treatment: Diffusion of impurities into the material (extended heating at 1800°C)
Specific process: Add Cr2 O3, NiO, etc
Result: Produces other colors
Diffusion treatment
Method Nos. 7-9 are completely different from the above because minute quantities of ‘trace elements’ are added during the process. Proof of such a treatment is important for more than commercial reasons, since these products must be labeled ‘treated corundum’ according to the rules of CIBJO. The introduction of the ‘traces’ is diffusion induced. At the high temperature of 1700°C, the crystal lattice of corundum is somewhat expanded. The atoms are more mobile and interatomic distances enlarged relative to the cold state. Under conditions near the melting point, the iron or titanium present in higher concentrations outside the crystal is able to travel a short distance into the crystal. The depth of this diffusion process is dependant on the temperature and the duration of the treatment. Partial melting at the surface is frequently brought about by the strong heating, as well as by the reduction in melting point temperature caused by ‘trace elements’. The polished faces of a stone are dotted with tiny pits and droplets and must be repolished.
However, the diffusion treatment only reaches superficial areas. By replolishing a diffusion-treated corundum, its former colorlessness may be brought out again.
Occasionally, diffusion-treated corundums are incorrectly termed ‘coated sapphires’. In contrast to beryl with an overgrowth of synthetic emerald (after Lechleitner), diffusion-treated corundum does not display a ‘cultivated’ layer, but a barely measurable quantity of color has been soaked by the surface.
The diffusion method is not only used for producing blue corundum but is also employed with other elements to produce red (chromium), padparadscha and other colors.
Features of heat treated corundums
The raw material from which the following features are described are sapphires from Sri Lanka. They form the bulk of treatable material which reaches a high quality. Initially, these sapphires were weak in color, partially interspersed with silk, colored in narrow bands only, or of a yellowish cloudiness. With the sudden and intense heating which usually last one to two days, tension fractures may develop. Endangered regions are located at material inhomogenities. Therefore, tension cracks will most probably develop around inclusions and at the surface. The differential dilations of mineral, gas, and fluid inclusions contribute additionally to the formation of fissures. This mechanism occurs in nature during the metamorphism of rocks. But due to the rapid rates of artificial heating, these inclusions differ from those seen in natural stones. This is evident when comparing pictures of untreated corundums (Gubelin, 1973) with those presented in this paper.
In addition to the change of the original inclusions, we recognize the development of new forms of inclusions, for example, altered zonal structures of possibly former rutile; white spheric aggregates with a thorny surface. These ‘ping-pong balls’ frequently are surrounded by disc-shaped tension fissures. They resemble the natural, structured healing fissures in untreated Sri Lankan sapphires. Normally, the subtle healing fissures similar to insect wings transform to more bulky forms as hoses or rows of droplets; may show a type of tension crack for corundum, but similar to those seen in peridot. These fissures are again disc-shaped, iridescent and resemble atolls. Frequently, the center contains a tiny grain. The fissures are smooth and structured at the periphery only. A further feature of heat treated corundums is the grainy, pockmarked surface.
Normally, all facets or parts of the girdle are overlooked. The repolished girdle frequently is composed of several steps.
Features of diffusion treated corundums
A dark ring rimming the stone near the girdle is the first recognizable characteristic in diffusion treated stones. This sign is visible with the naked eye. In this zone, the stone is thin and the effect of the diffusion-layer is more prominent. In the center, we find in many cases a ‘hole’ of color. Here the effect of the diffusion layer is weakest. The girdle may appear colorless if the diffusion layer has been removed totally when repolished. Damaged spots and small pits in the surface of a pre-cut stone ready for treatment facilitate reinforced absorption of the coloring elements. A crack, i.e contact of two surfaces leads to a double diffusion layer, and therefore to a stronger color reception. Also concentrations of color in depressions are frequently encountered.
The recognition of corundum colored by a diffusion process should not present great difficulties. In most cases it will suffice to look at the stones in front of a diffusely bright background when they are immersed in a liquid with a high refractive index. At low magnification, the stronger colored facet edges are easily visible. Also, due to unequal forces applied during the repolishing, the differences in saturation of the individual facets are visible in immersion.
For the past two years blue sapphires with good color and pleasing size have become more abundant in the trade. Surprisingly, these stones are offered in lots in which the individual stones are uniformly colored. This is not common with larger gems of high value. Initially, we dealt with such sapphires very carefully and objectively due to a vague suspicion.
During the course of our investigation (Bosshart 1981) the subject of heat treatment of corundum has been brought up at various international meetings of gemologists. Finally, Nassau, Crowningshield and Liddicoat went public with some presentations, which if not quite complete are fairly comprehensive. This report is supplementary state of the author to a recent publication (Hanni 1982).
Heat treatment of corundum has long been practiced, on the one hand after indigenous methods in the countries of origin (i.e. Sri Lanka), on the other hand after an industrial method employed since 1973. With local methods, controlling of temperature and environment is not satisfactorily possible. Because of this, the results are unreliable. Application of the industrial method, carried out in the USA and Thailand, guarantees constant, controlled conditions and leads to the desired results. Let us consider the patents of Union Carbide in 1973 as a starting point. In these patents, techniques are described which allow (at first applied to synthetics only) a homogenization or improvement of the color or the development of asterism. Over the last years, these processes have been tried and improved on natural corundum. Presently, masters of such techniques reside in Thailand. Their latest surprising creations are yellow to orange corundums produced by heat treating raw material corundum of an as yet undescribed type.
The aim of heat treatment is to reach a uniform distribution of color, to clarify or to increase the color, or to attain better purity by dissolving certain inclusions. Critical is whether these stones have absorbed foreign matter during the process of heat treatment, or the required improvement was achieved with those components already present in the stone. Recently, Dr Nassau has reviewed the different processes (Nassau 1981, tab 1). These nine types of treatment may be divided into two groups: With Nos. 1-6, a simple heating occurs; with Nos. 7-9, color producing chemicals are added during the process. Case No.6 may be regarded separately, since with this treatment, natural-looking fingerprint inclusions are generated in synthetic stones.
Heat treatment
This represents a normal annealing (but an extremely high temperature), a process which is well-known and applied to amethyst, zircon, topaz, tourmaline, etc. The new color is considered to be stable. Corundum treated after the processes Nos. 1-5 may be traded without complementary designation according to the regulations of CIBJO colored stones commission (CIBJO 1981).
For example, process No. 3 can be demonstrated with blue sapphire as follows: iron and titanium in the form of Fe/Ti-pairs in the corundum crystal lattice are responsible for the blue color in sapphires (Schmetzer & Bank, 1981). If one of the two partners is less abundant, its deficiency cannot be compensated by addition of the other partner. The solubility of iron and titanium is higher at high temperatures than at low temperatures. If a corundum saturated in Fe and Ti cools slowly, TiO2 is able to unmix from the corundum lattice as rutile (or silk). In this way, Ti is eliminated as a color producing component. On the other hand, by reductive heating a light blue rutile containing corundum to 1600°C and cooling it rapidly, one may obtain stones with intense blue color and a better transparency. The rutile has dissolved and joined with the possibly present iron to form the color yielding Fe/Ti pairs (Fe²+ / Ti4+ charge transfer).
Heat treatment processes used on sapphires and rubies (after K Nassau, 1981)
1. Treatment: Heating only
Specific process: Moderate temperature (1300°C)
Result: Develops potential asterism
2. Treatment: Heating only
Specific process: High temperature (1600°C) / rapid cooling
Result: Removes silk and asterism
3. Treatment: Heating only
Specific process: Reductive heating (1600°C)
Result: Develops potential blue color
4. Treatment: Heating only
Specific process: Oxidative heating (1600°C)
Result: Diminishes blue color
5. Treatment: Heating only
Specific process: Extended heating (1800°C)
Result: Diminishes Verneuil banding and strain
6. Treatment: Heating under unknown conditions (this process is used on synthetic material to generate inclusions with a natural appearance)
Specific process: ?
Result: Introduces fingerprint inclusions
7. Treatment: Diffusion of impurities into the material (extended heating at 1800°C)
Specific process: Add TiO2
Result: Produces asterism
8. Treatment: Diffusion of impurities into the material (extended heating at 1800°C)
Specific process: Add TiO2 and / or Fe2 O3
Result: Produces blue color
9. Treatment: Diffusion of impurities into the material (extended heating at 1800°C)
Specific process: Add Cr2 O3, NiO, etc
Result: Produces other colors
Diffusion treatment
Method Nos. 7-9 are completely different from the above because minute quantities of ‘trace elements’ are added during the process. Proof of such a treatment is important for more than commercial reasons, since these products must be labeled ‘treated corundum’ according to the rules of CIBJO. The introduction of the ‘traces’ is diffusion induced. At the high temperature of 1700°C, the crystal lattice of corundum is somewhat expanded. The atoms are more mobile and interatomic distances enlarged relative to the cold state. Under conditions near the melting point, the iron or titanium present in higher concentrations outside the crystal is able to travel a short distance into the crystal. The depth of this diffusion process is dependant on the temperature and the duration of the treatment. Partial melting at the surface is frequently brought about by the strong heating, as well as by the reduction in melting point temperature caused by ‘trace elements’. The polished faces of a stone are dotted with tiny pits and droplets and must be repolished.
However, the diffusion treatment only reaches superficial areas. By replolishing a diffusion-treated corundum, its former colorlessness may be brought out again.
Occasionally, diffusion-treated corundums are incorrectly termed ‘coated sapphires’. In contrast to beryl with an overgrowth of synthetic emerald (after Lechleitner), diffusion-treated corundum does not display a ‘cultivated’ layer, but a barely measurable quantity of color has been soaked by the surface.
The diffusion method is not only used for producing blue corundum but is also employed with other elements to produce red (chromium), padparadscha and other colors.
Features of heat treated corundums
The raw material from which the following features are described are sapphires from Sri Lanka. They form the bulk of treatable material which reaches a high quality. Initially, these sapphires were weak in color, partially interspersed with silk, colored in narrow bands only, or of a yellowish cloudiness. With the sudden and intense heating which usually last one to two days, tension fractures may develop. Endangered regions are located at material inhomogenities. Therefore, tension cracks will most probably develop around inclusions and at the surface. The differential dilations of mineral, gas, and fluid inclusions contribute additionally to the formation of fissures. This mechanism occurs in nature during the metamorphism of rocks. But due to the rapid rates of artificial heating, these inclusions differ from those seen in natural stones. This is evident when comparing pictures of untreated corundums (Gubelin, 1973) with those presented in this paper.
In addition to the change of the original inclusions, we recognize the development of new forms of inclusions, for example, altered zonal structures of possibly former rutile; white spheric aggregates with a thorny surface. These ‘ping-pong balls’ frequently are surrounded by disc-shaped tension fissures. They resemble the natural, structured healing fissures in untreated Sri Lankan sapphires. Normally, the subtle healing fissures similar to insect wings transform to more bulky forms as hoses or rows of droplets; may show a type of tension crack for corundum, but similar to those seen in peridot. These fissures are again disc-shaped, iridescent and resemble atolls. Frequently, the center contains a tiny grain. The fissures are smooth and structured at the periphery only. A further feature of heat treated corundums is the grainy, pockmarked surface.
Normally, all facets or parts of the girdle are overlooked. The repolished girdle frequently is composed of several steps.
Features of diffusion treated corundums
A dark ring rimming the stone near the girdle is the first recognizable characteristic in diffusion treated stones. This sign is visible with the naked eye. In this zone, the stone is thin and the effect of the diffusion-layer is more prominent. In the center, we find in many cases a ‘hole’ of color. Here the effect of the diffusion layer is weakest. The girdle may appear colorless if the diffusion layer has been removed totally when repolished. Damaged spots and small pits in the surface of a pre-cut stone ready for treatment facilitate reinforced absorption of the coloring elements. A crack, i.e contact of two surfaces leads to a double diffusion layer, and therefore to a stronger color reception. Also concentrations of color in depressions are frequently encountered.
The recognition of corundum colored by a diffusion process should not present great difficulties. In most cases it will suffice to look at the stones in front of a diffusely bright background when they are immersed in a liquid with a high refractive index. At low magnification, the stronger colored facet edges are easily visible. Also, due to unequal forces applied during the repolishing, the differences in saturation of the individual facets are visible in immersion.
Artificial Asterism
(via Gemmology Queensland, Vol.3, No.2, February 2002/ GZ English language translation) Dr Karl Schmetzer / Martin P Steinbach writes:
A new method for synthetically altering precious gems was discovered only recently. In this case it is a method of synthetically creating asterism in natural materials, cut as a cabochon. Up until now, the creation of synthetic asterism, e.g. by diffusion treatment of sapphires, was only applied to precious gems if these precious gems also produced natural star stones.
A new study by S F McClure and J I Koivula, titled A new method of imitating asterism in Gems & Gemology, Vol. 37, No.2, 2001, pp 124-128, now describes precious gem minerals with synthetic asterism, although the gems themselves (sinhalite, cassiterite and samarskite) do not produce star stones naturally. Synthetic asterism is also described for minerals, which have produced natural star stones such as garnet, chrysoberyl or rutile.
According to McClure and Koivula, the new type of synthetic asterism is created by orientated series of scratches on the surfaces of the precious gems, cut as cabochon, in which the scratches run parallel to each other. Although details of the treatment method are as yet unknown, it is assumed that the scratches are applied to the surface of the cabochon by hand, whereby the number of series of parallel scratches defines the number of arms in the synthetic stars.
Although it has been known since the 19th century that asterism can be created in metal plates, for example, through oriented scratches, this method had not yet been applied to natural precious gems in order to create synthetic asterism. A detailed treatment of the topic of asterism on even metal sheets is contained in a study penned by W Maier titled Experimental Asterism in the New Journal for Mineralogy, Geology and Paleontology, Vol.78, part 3, 1943, pp 283-380. This article also describes two other examples of synthetic asterism in natural precious gems.
The author purchased the two cabochons from a dealer. The seller claimed that the stones originated from India or Sri Lanka. The first cabochon with synthetic asterism that was examined was a reddish brown cabochon of 3.08 ct. Absorption spectroscopy revealed a spectrum with a series of iron bands, which are typical for the members of the mixed crystal family pyrope/almandine garnet. The microscopic examination of the transparent, actually very pure stone reveals only a few rutile needles, running at a slant to the surface of the cabochon. The orientation and in particular the low number of these rutile needles meant that they most certainly did not contribute to the asterism observed in this stone. The star itself is made up of nine (9) sharp lines of light, caused by nine series of parallel scratches on the surface of the garnet cabochon. In general, four-arm asterism and more rarely six-arm asterism have been described as occurring naturally in various deposits of garnet. A star with nine arms is incompatible with the cubic symmetry of garnet.
The second ‘star’ precious gem examined was an opaque, black tourmaline of 15.04 ct. Its relatively high refractive indices of 1.625 and 1.646 revealed the black tourmaline must be a stone with relatively high ferrous iron content. The asterism observed in this stone consisted of a six-armed star. The pattern and symmetry of the six arms of the star was homogenous with the trigonal symmetry of tourmaline. However, one additional ‘satellite’ could be observed on one of these arms. It ran from the center of the cabochon alongside the main arm to approximately the middle of the cabochon. A second, less distinct, additional line also ran six main lines. Parallel stripes and scratches could also be observed on the surface of the cabochon; and these were responsible for the synthetic asterism.
At the moment—that is in the stones observed in retail trade up until now, the determination and identification of this kind of manipulation or treatment method to create synthetic asterism has been relatively simple.
A certain number of light lines and/or a symmetry in the star that does not conform to the symmetry of the precious stone itself on which the observed asterism was created synthetically, speaks clearly for manipulation and not for natural asterism. The existence of incomplete arms and light lines with ‘false’ orientation, also known as ‘satellites’ of main arms of the star, also denote that the star was created artificially. The same applies to the parallel scratches that can be observed on the surfaces of the manipulated cabochon under the microscope, if the stones do not have in their centers any ‘needles’ with an orientation and concentration required for the natural creation of a star.
A new method for synthetically altering precious gems was discovered only recently. In this case it is a method of synthetically creating asterism in natural materials, cut as a cabochon. Up until now, the creation of synthetic asterism, e.g. by diffusion treatment of sapphires, was only applied to precious gems if these precious gems also produced natural star stones.
A new study by S F McClure and J I Koivula, titled A new method of imitating asterism in Gems & Gemology, Vol. 37, No.2, 2001, pp 124-128, now describes precious gem minerals with synthetic asterism, although the gems themselves (sinhalite, cassiterite and samarskite) do not produce star stones naturally. Synthetic asterism is also described for minerals, which have produced natural star stones such as garnet, chrysoberyl or rutile.
According to McClure and Koivula, the new type of synthetic asterism is created by orientated series of scratches on the surfaces of the precious gems, cut as cabochon, in which the scratches run parallel to each other. Although details of the treatment method are as yet unknown, it is assumed that the scratches are applied to the surface of the cabochon by hand, whereby the number of series of parallel scratches defines the number of arms in the synthetic stars.
Although it has been known since the 19th century that asterism can be created in metal plates, for example, through oriented scratches, this method had not yet been applied to natural precious gems in order to create synthetic asterism. A detailed treatment of the topic of asterism on even metal sheets is contained in a study penned by W Maier titled Experimental Asterism in the New Journal for Mineralogy, Geology and Paleontology, Vol.78, part 3, 1943, pp 283-380. This article also describes two other examples of synthetic asterism in natural precious gems.
The author purchased the two cabochons from a dealer. The seller claimed that the stones originated from India or Sri Lanka. The first cabochon with synthetic asterism that was examined was a reddish brown cabochon of 3.08 ct. Absorption spectroscopy revealed a spectrum with a series of iron bands, which are typical for the members of the mixed crystal family pyrope/almandine garnet. The microscopic examination of the transparent, actually very pure stone reveals only a few rutile needles, running at a slant to the surface of the cabochon. The orientation and in particular the low number of these rutile needles meant that they most certainly did not contribute to the asterism observed in this stone. The star itself is made up of nine (9) sharp lines of light, caused by nine series of parallel scratches on the surface of the garnet cabochon. In general, four-arm asterism and more rarely six-arm asterism have been described as occurring naturally in various deposits of garnet. A star with nine arms is incompatible with the cubic symmetry of garnet.
The second ‘star’ precious gem examined was an opaque, black tourmaline of 15.04 ct. Its relatively high refractive indices of 1.625 and 1.646 revealed the black tourmaline must be a stone with relatively high ferrous iron content. The asterism observed in this stone consisted of a six-armed star. The pattern and symmetry of the six arms of the star was homogenous with the trigonal symmetry of tourmaline. However, one additional ‘satellite’ could be observed on one of these arms. It ran from the center of the cabochon alongside the main arm to approximately the middle of the cabochon. A second, less distinct, additional line also ran six main lines. Parallel stripes and scratches could also be observed on the surface of the cabochon; and these were responsible for the synthetic asterism.
At the moment—that is in the stones observed in retail trade up until now, the determination and identification of this kind of manipulation or treatment method to create synthetic asterism has been relatively simple.
A certain number of light lines and/or a symmetry in the star that does not conform to the symmetry of the precious stone itself on which the observed asterism was created synthetically, speaks clearly for manipulation and not for natural asterism. The existence of incomplete arms and light lines with ‘false’ orientation, also known as ‘satellites’ of main arms of the star, also denote that the star was created artificially. The same applies to the parallel scratches that can be observed on the surfaces of the manipulated cabochon under the microscope, if the stones do not have in their centers any ‘needles’ with an orientation and concentration required for the natural creation of a star.
Wednesday, April 18, 2007
Langasite
(via Gemmology Queensland, Vol 3, No.3, March 2002)
Langasite is the name given to a crystal-pulled (Czochralski grown) lanthium gallium silicate. Although designed for use in the communications industry this synthetic material has properties that make it suitable for use as a faceting material.
The material is transparent, dispersive, and of yellow orange to orange color (depending on the oxygen content of the atmosphere in which the crystal is grown).
Langasite has the following identifying properties:
Crystal system: Trigonal
Color: Yellow orange to orange
Hardness: 6 -7
Luster: Vitreous
Diaphaneity: Transparent
Specific gravity: 4.65
Refractive indices: 1.910 – 1.921
Luminescence: Inert
Absorption spectrum: No identifying absorptions
Inclusions: Occasional solid inclusions of triangular shape, rare two phase inclusions.
The only similarly colored gemstone this synthetic is likely to be confused with is zircon (SG=4.70; RI=1.92-1.98). But the characteristic absorption spectrum of zircon (strong line at 653.5nm) should allow effective discrimination between the two.
Langasite is the name given to a crystal-pulled (Czochralski grown) lanthium gallium silicate. Although designed for use in the communications industry this synthetic material has properties that make it suitable for use as a faceting material.
The material is transparent, dispersive, and of yellow orange to orange color (depending on the oxygen content of the atmosphere in which the crystal is grown).
Langasite has the following identifying properties:
Crystal system: Trigonal
Color: Yellow orange to orange
Hardness: 6 -7
Luster: Vitreous
Diaphaneity: Transparent
Specific gravity: 4.65
Refractive indices: 1.910 – 1.921
Luminescence: Inert
Absorption spectrum: No identifying absorptions
Inclusions: Occasional solid inclusions of triangular shape, rare two phase inclusions.
The only similarly colored gemstone this synthetic is likely to be confused with is zircon (SG=4.70; RI=1.92-1.98). But the characteristic absorption spectrum of zircon (strong line at 653.5nm) should allow effective discrimination between the two.
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