(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.
Friday, April 20, 2007
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.
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