(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?”
Friday, April 20, 2007
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.
More On Coated Topaz
(via Gemmology Queensland, Vol 3, No.6, July 2002)
Blue, bluish green, and green surface-diffused topaz that are produced in the USA by heat diffusion of a cobalt rich (› blue) or cobalt and nickel rich (› green) powders, have been marketed since 1998. The color, which is uniform to the naked eye, is confined to a thin layer on or jut below the surface of the treated colorless topaz. When examined (immersed) under magnification the diffused layer has a patchy color distribution. Chipped facets reveal underlying colorless topaz. This coating may produce anomalous (› 1.81) reading with the gem refractometer. When examined with the VIS spectroscope, cobalt absorption bands at 560, 590 and 640nm are visible, and this color-enhanced topaz commonly gives a red response when examined with the Chelsea filter.
Well….a new coated topaz has appeared on the market. This treated topaz, that has a brownish ‘imperial’ topaz color was created by Prof Vladimir Balitsky of the Mineralogical Institute, Russian Academy of Sciences. Details of this new treatment were revealed in his recent lecture to The Gemmological Association of All Japan.
The starting material is faceted F-bearing colorless topaz that is coated with a thin layer of iron oxide by heat diffusion. This thin coating gives the topaz a brownish yellow color. When the coating is examined in tangential illumination the facet displays a high, iridescent luster. As the coating is comparatively soft, it is readily abraded and chipped thus exposing the underlying colorless topaz. Refractive indices from the coated facets were 1.61 – 1.62 (normal for F-topaz, including yellow or brown topaz), but the coated topaz did not display the orange LWUV fluorescence normally displayed by imperial (OH-type) topaz.
The Professor remarked that hydrothermally grown synthetic topaz will be soon released on to the world market.
Blue, bluish green, and green surface-diffused topaz that are produced in the USA by heat diffusion of a cobalt rich (› blue) or cobalt and nickel rich (› green) powders, have been marketed since 1998. The color, which is uniform to the naked eye, is confined to a thin layer on or jut below the surface of the treated colorless topaz. When examined (immersed) under magnification the diffused layer has a patchy color distribution. Chipped facets reveal underlying colorless topaz. This coating may produce anomalous (› 1.81) reading with the gem refractometer. When examined with the VIS spectroscope, cobalt absorption bands at 560, 590 and 640nm are visible, and this color-enhanced topaz commonly gives a red response when examined with the Chelsea filter.
Well….a new coated topaz has appeared on the market. This treated topaz, that has a brownish ‘imperial’ topaz color was created by Prof Vladimir Balitsky of the Mineralogical Institute, Russian Academy of Sciences. Details of this new treatment were revealed in his recent lecture to The Gemmological Association of All Japan.
The starting material is faceted F-bearing colorless topaz that is coated with a thin layer of iron oxide by heat diffusion. This thin coating gives the topaz a brownish yellow color. When the coating is examined in tangential illumination the facet displays a high, iridescent luster. As the coating is comparatively soft, it is readily abraded and chipped thus exposing the underlying colorless topaz. Refractive indices from the coated facets were 1.61 – 1.62 (normal for F-topaz, including yellow or brown topaz), but the coated topaz did not display the orange LWUV fluorescence normally displayed by imperial (OH-type) topaz.
The Professor remarked that hydrothermally grown synthetic topaz will be soon released on to the world market.
Chinese Pearl Enhancement Techniques
(Gemmology Queensland, Volume 3, No.6, June 2002)
According to Guo & Shi on pages 32-36 of Volume XXII of The Journal of the Gemmological Association of Hong Kong, Chinese pearl processors apply a range of enhancement technology to both their freshwater and saltwater cultured pearls. These routine treatments include:
- Pre treatment
- Bleaching
- Increasing whiteness
- Adding luster
- Dyeing
- Polishing
Pre Treatment
Pre-treatment process includes sorting, drilling, expansion and dehydration steps. Pearls are sorted on the basis of thickness of nacre, color, luster, shape and size. Drilling allows stringing and setting removes some external defects, and is an essential prerequisite for subsequent treatments for reducing oil content, bleaching, adding whiteness and dyeing pearls. Expansion looses the pearl’s structure and so facilitates future treatment. Two techniques of expansion are used at a temperature of 70-80°C and for times that depend on the depth of color of the pearl (darker color, more time). These techniques of expansion are:
- Heating pearls that have been wrapped in gauze in deionized water.
- Immersion of pearls in an unspecified liquid.
- Dehydration follows expansion. The removal of water from cracks in the pearl is accomplished by immersion in anhydrous ethanol and glycerol.
Bleaching
Bleaching is used to create white pearls and to remove disfiguring colors and stains from the pearls. The common bleaching solution consists of a mixture of hydrogen peroxide, a solvent of surface active reagent (detergent), and a pH stability regulator. Factors affecting the efficiency of bleaching include concentration of hydrogen peroxide, temperature pH of bleaching solution, composition of surface active agent, pH stabilizer and solvent duration of light exposure, and the frequency of stirring and renewal of the bleaching solution.
Two formulations are commonly used for bleaching pearls:
- Hydrogen peroxide carbinol ammonia, 12-alkyl sodium sulphate, Britton-Robinson buffer solvent.
- Hydrogen peroxide chloroform, ethanol amine, 12-alkyl sodium sulphate, Britton-Robinson buffer solvent.
- Common bleaching conditions are a temperature of 30-35°C in association with light exposure. Hydrogen peroxide of 2-3% is commonly used, while bleaching times range from 2-3 days for light colored pearls to more than 10 days for darker colored pearls. The bleaching solution is changed every four days.
Increasing Whiteness
Following bleaching some pearls still retail a yellowish tone due to the presence of pigments resistant to the bleaching effect of hydrogen peroxide. These pearls are further treated with a fluorescent whitener. This is a dye molecule that is activated by the UV in visible light to emit a bluish fluorescence which neutralizes the residual yellow color of pearl and produces a whiter pearl. A Swiss manufactured whitener FP, dissolved in acetone and the surface active agents acrylic acid for 12-alkyl sodium sulphate is the fluorescence whitener commonly used on Chinese pearls.
Adding Luster
This is claimed to be one of the most important processes in pearl enhancement. It involves immersing the pearls in a weakly alkaline TGP (magnesium compounds) solution and keeping them constantly stirred at even temperature for several days.
Dyeing
As bleached pearls usually have some residual uneven color, they are dyed to yield fashionable and marketable colors. Specific dyes are used to produce predictable colors, e.g. potassium permanganate solution with golden NH-R yields golden pearls, while Luo Dan Ming B in alcohol produces pink pearls. These dyes can be either water, oil or alcohol based.
Polishing
Polishing, the last step enhances both the smoothness and luster of the pearls. This step involves surface polishing, usually by tumbling in a mild abrasive, followed by coating of the pearls with a thin layer of wax.
The Problem
- Which of these treatments should be detected?
- Which of these treatments should be disclosed?
According to Guo & Shi on pages 32-36 of Volume XXII of The Journal of the Gemmological Association of Hong Kong, Chinese pearl processors apply a range of enhancement technology to both their freshwater and saltwater cultured pearls. These routine treatments include:
- Pre treatment
- Bleaching
- Increasing whiteness
- Adding luster
- Dyeing
- Polishing
Pre Treatment
Pre-treatment process includes sorting, drilling, expansion and dehydration steps. Pearls are sorted on the basis of thickness of nacre, color, luster, shape and size. Drilling allows stringing and setting removes some external defects, and is an essential prerequisite for subsequent treatments for reducing oil content, bleaching, adding whiteness and dyeing pearls. Expansion looses the pearl’s structure and so facilitates future treatment. Two techniques of expansion are used at a temperature of 70-80°C and for times that depend on the depth of color of the pearl (darker color, more time). These techniques of expansion are:
- Heating pearls that have been wrapped in gauze in deionized water.
- Immersion of pearls in an unspecified liquid.
- Dehydration follows expansion. The removal of water from cracks in the pearl is accomplished by immersion in anhydrous ethanol and glycerol.
Bleaching
Bleaching is used to create white pearls and to remove disfiguring colors and stains from the pearls. The common bleaching solution consists of a mixture of hydrogen peroxide, a solvent of surface active reagent (detergent), and a pH stability regulator. Factors affecting the efficiency of bleaching include concentration of hydrogen peroxide, temperature pH of bleaching solution, composition of surface active agent, pH stabilizer and solvent duration of light exposure, and the frequency of stirring and renewal of the bleaching solution.
Two formulations are commonly used for bleaching pearls:
- Hydrogen peroxide carbinol ammonia, 12-alkyl sodium sulphate, Britton-Robinson buffer solvent.
- Hydrogen peroxide chloroform, ethanol amine, 12-alkyl sodium sulphate, Britton-Robinson buffer solvent.
- Common bleaching conditions are a temperature of 30-35°C in association with light exposure. Hydrogen peroxide of 2-3% is commonly used, while bleaching times range from 2-3 days for light colored pearls to more than 10 days for darker colored pearls. The bleaching solution is changed every four days.
Increasing Whiteness
Following bleaching some pearls still retail a yellowish tone due to the presence of pigments resistant to the bleaching effect of hydrogen peroxide. These pearls are further treated with a fluorescent whitener. This is a dye molecule that is activated by the UV in visible light to emit a bluish fluorescence which neutralizes the residual yellow color of pearl and produces a whiter pearl. A Swiss manufactured whitener FP, dissolved in acetone and the surface active agents acrylic acid for 12-alkyl sodium sulphate is the fluorescence whitener commonly used on Chinese pearls.
Adding Luster
This is claimed to be one of the most important processes in pearl enhancement. It involves immersing the pearls in a weakly alkaline TGP (magnesium compounds) solution and keeping them constantly stirred at even temperature for several days.
Dyeing
As bleached pearls usually have some residual uneven color, they are dyed to yield fashionable and marketable colors. Specific dyes are used to produce predictable colors, e.g. potassium permanganate solution with golden NH-R yields golden pearls, while Luo Dan Ming B in alcohol produces pink pearls. These dyes can be either water, oil or alcohol based.
Polishing
Polishing, the last step enhances both the smoothness and luster of the pearls. This step involves surface polishing, usually by tumbling in a mild abrasive, followed by coating of the pearls with a thin layer of wax.
The Problem
- Which of these treatments should be detected?
- Which of these treatments should be disclosed?
Subscribe to:
Comments (Atom)