(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.
Friday, April 20, 2007
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?
Stanthorpe’s Green Diamond
(via Gemmology Queensland, Volume 3, No.9, September 2002 / The Queensland Government Mining Journal of 15th April, 1924)
A Sad Tale
In a letter to the Brisbane Daily Mail, Mr Oscar Meston (Mining Warden at Stanthorpe) writes:
“On 10th March a cable from London stated that the only known green diamond in existence would be exhibited at the Wembley Exhibition. It weighs a carat and a half, is worth £1750, and was found in the Transvaal in 1923. The Stanthorpe mineral field has produced a rival to this remarkable gem. A few years ago my eye was attracted by ‘coruscation’ from a heap of tailings on an abandoned tin claim. I discovered the source of the light to be a small green stone, which I immediately recognized as a diamond. It was a perfect octahedron, pale green in color, flawless, and a limpid beauty—and it weighed two carats and a half. The surface showed very fine striations, evidently caused by movement under enormous pressure. I offered the stone to several gem merchants in Sydney, but found them almost as adamant as the diamond itself. Their chief objection was the excessive hardness of the Australian diamond and consequent lapidarian difficulties. The utmost I could obtain for the stone was 20 guineas. On the basis of valuation of the Transvaal stone, I lost approximately £2900 on the transaction.”
A Sad Tale
In a letter to the Brisbane Daily Mail, Mr Oscar Meston (Mining Warden at Stanthorpe) writes:
“On 10th March a cable from London stated that the only known green diamond in existence would be exhibited at the Wembley Exhibition. It weighs a carat and a half, is worth £1750, and was found in the Transvaal in 1923. The Stanthorpe mineral field has produced a rival to this remarkable gem. A few years ago my eye was attracted by ‘coruscation’ from a heap of tailings on an abandoned tin claim. I discovered the source of the light to be a small green stone, which I immediately recognized as a diamond. It was a perfect octahedron, pale green in color, flawless, and a limpid beauty—and it weighed two carats and a half. The surface showed very fine striations, evidently caused by movement under enormous pressure. I offered the stone to several gem merchants in Sydney, but found them almost as adamant as the diamond itself. Their chief objection was the excessive hardness of the Australian diamond and consequent lapidarian difficulties. The utmost I could obtain for the stone was 20 guineas. On the basis of valuation of the Transvaal stone, I lost approximately £2900 on the transaction.”
Every One Needs A 10x
(How to) Use it seriously
(via Gemmology Queensland, No.3. No 6, July 2002) Trevor Linton writes:
Carry it all the time and use it as much as possible every day. A 10x lens increases knowledge. A 10x is the first and possibly the most serious instrument used by both the amateur and professional when investigating a gemstone’s features and information as to its origin.
The hand lens, (10x or times—10) is a combination of lenses that provides magnified detail to the observer. It is the most important instrument you can use when an unknown gemstone is given to you either for purchase or for identification. Do not dismiss this step of the investigation into a gemstone during your quest for information, as the hand lens is the only instrument that provides an overall view of a gemstone. The 10x hand lens, in experienced hands, and under quality lighting, can provide essential identification features on 90 percent of gemstones: by noting hardness, refractive index, birefringence, dichroism and dispersion as well as indications of the gemstone’s origin from inclusions and many methods of man’s treatment.
The quality of a 10x hand lens is usually dependant on price, which can vary from A $10 to $320 depending on its type, source, manufacturer and country of origin. There are three main lens combinations that govern the quality of the lens: the plano-convex doublet, the achromatic doublet, and the compound triplet. Each has its advantages and disadvantages.
The first type, the economic doublet has two simple crown glass lenses facing each other at approximately two thirds of their focal length. This combination of lenses provides a 40 percent field of vision that is in reasonable focus and has a relatively flat field. When observing a sheet of squared paper with the doublet, an image appears. This is suitable for rough investigation of stones, but when serious images of a flat facet are required, it leaves a lot to be desired……as straight lines are curved, only the center is in focus as the observed field is curved through a doublet. The colors red and blue are not in focus with green. The advantage for the doublet is economy. A lot of detail can be gathered for a small outlay of dollars with a doublet. Chromatic aberration is usually not a great influence with gemstones as only highlights are refracted in to the differing points of color focus that produce color fringing. Most gemstone observations are in gemstones of one color or colorless gemstones.
Increased color resolution is achieved with a flint glass lens cemented to the inside a double sided convex crown glass lens forming an achromatic compound lens that corrects for the chromatic aberrations that defocus colors in a lens system. This second type of lens system produces straight lines over most of the field of view. An achromatic doublet is a very good, yet still economic 10x system of lenses.
The third lens type, the triplet lens, is a solid system of glass lenses using three layers of glass cemented together forming an aplanatic (in a plane) triplet. The image of a flat plane forms as a plane at the point of focus. This is the best system for hand lenses, when correctly designed and used under normal observation. A triplet has minimal spherical and chromatic aberration and a flat field.
When purchasing a hand lens, buy one that is better than you initially need. If you buy the best triplet lens, use it to your advantage. That s what the whole project should be about. You have to achieve quality observation. There is little advantage in using a quality lens without good lighting and knowledge of how to use both.
When using the hand lens there are several basic rules, and many techniques to develop:
- Spectacles should not be worn unless severe optical defects are present within eyes.
- Choose a good quality light source, with a very narrow beam of high intensity light directed on to your work area and not spilling light towards your eye or the background.
- Work over a soft white cloth that allows easy pick up of gemstones with tweezers. This soft cloth also prevents dropped gemstones from bouncing off the work area on to the floor.
- Clean the sample free from dust and grease (fingerprints) with a lint free cloth. A quality glasses cleaning cloth is suitable.
- Always pick up gemstones with tweezers after cleaning.
- When using the right eye hold the hand lens in the right hand about 25mm in front of the eye, with the index finger primary knuckle resting on the neck.
- Keep both eyes open. This can occur with experience. When the closed eye lid flutters letting light in there is a complimentary iris movement in the open eye.
- Holding the lens close to the eye with one hand, bring the gemstone close with the other hand and rest both hands together for stability. Position the stone 25mm in front of the lens with the gem’s surface in focus, under the light.
- Ensure the lens is straight in front of your eye and is not twisted. Looking off the central axis will produce ‘coma’ color fringing on all images that severely limit quality observing.
- Work systematically around the surface of all faceted gemstones before being drawn into the gems interior. There are many features available for observation, especially around the girdle. Gemstones such as diamond reveal their true nature on the girdle, with natural crystal surfaces left on opposite side of the girdle as the cutters try to achieve maximum weight from the rough stone. The treatment of the girdle or how it was formed is a good guide to the cutting quality and whether care was taken during the gem’s manufacture.
- Do not neglect other features such as flat facets and sharp facet edges that indicate hard gemstones. Concave facets or molding marks at the girdle indicate cast glass. Reflect a fluorescent tube image from the facet for an image of a straight flat facet. Surface luster of facet edges chips indicates the refractive index of the gemstone. Try looking at glass or quartz and comparing surface chips with those on sapphire or diamond. Yes diamond does chip before it cleaves in to two diamonds.
The history of a gemstone’s growth, and subsequent heat and coloring activities induced by man, occurs in the gem’s inclusions. Your 10x hand lens will reveal many of these features under these proposed lighting techniques of use.
Inclusion types have a greater listing than there are gemstone types. Quartz alone has 550 officially listed inclusion types. Quartz is a low temperature forming gemstone; it is of the last to crystallize and includes many other gems that form before it does. Pegmatites in which quartz forms have plenty of liquid and gas so there are many of these inclusions in the ‘veils’ that form within fractures of quartz crystal.
Igneous (volcanic) gemstones can be easily distinguished from metamorphic gems by the individual suites of inclusions within each type. Man-made gems may be chemically identical with those of nature, but man’s techniques of manufacturing these gems create characteristic inclusions that are identifiable with your 10x hand lens.
An interesting feature is found on many older gemstones when a flat plane of air bubbles are glued in a layer between the crown and pavilion of a doublet. These are very easily found with a hand lens. Dispersion of color at individual facets is a good indicator of high refractive index and hardness.
Other hand lenses in use through the industry, such as the Coddington lens and the darkfield loupe have specific features for overcoming specific problems. For example, The Coddington lens is a solid glass cylinder with a restricting light aperture half way at its focal point. This light restriction reduces many aberration errors by preventing them passing the restriction. The darkfield loupe provides correct darkfield lighting for identifying small inclusions in gemstones and direction specific illumination on fracture fill inclusions. This is a small pocket portable, torch based instrument that has power full application in the diamond market of Europe, USA and Asia.
Recommendations
- Use your 10x for it can increase your knowledge and understanding of gemstones.
- A 10x lens is so much faster to use than a microscope that requires more detailed observation. Wear it out as soon as possible and buy a better quality lens as a Christmas present to yourself. If an excuse is needed, it will improve your observing and self-confidence. Do no think that the old 10x will do for the time being.
- Your 10x lens will repay with valuable information.
- Understanding information from a 10x depends on experience gained from informed and correct use f your most important instrument.
(via Gemmology Queensland, No.3. No 6, July 2002) Trevor Linton writes:
Carry it all the time and use it as much as possible every day. A 10x lens increases knowledge. A 10x is the first and possibly the most serious instrument used by both the amateur and professional when investigating a gemstone’s features and information as to its origin.
The hand lens, (10x or times—10) is a combination of lenses that provides magnified detail to the observer. It is the most important instrument you can use when an unknown gemstone is given to you either for purchase or for identification. Do not dismiss this step of the investigation into a gemstone during your quest for information, as the hand lens is the only instrument that provides an overall view of a gemstone. The 10x hand lens, in experienced hands, and under quality lighting, can provide essential identification features on 90 percent of gemstones: by noting hardness, refractive index, birefringence, dichroism and dispersion as well as indications of the gemstone’s origin from inclusions and many methods of man’s treatment.
The quality of a 10x hand lens is usually dependant on price, which can vary from A $10 to $320 depending on its type, source, manufacturer and country of origin. There are three main lens combinations that govern the quality of the lens: the plano-convex doublet, the achromatic doublet, and the compound triplet. Each has its advantages and disadvantages.
The first type, the economic doublet has two simple crown glass lenses facing each other at approximately two thirds of their focal length. This combination of lenses provides a 40 percent field of vision that is in reasonable focus and has a relatively flat field. When observing a sheet of squared paper with the doublet, an image appears. This is suitable for rough investigation of stones, but when serious images of a flat facet are required, it leaves a lot to be desired……as straight lines are curved, only the center is in focus as the observed field is curved through a doublet. The colors red and blue are not in focus with green. The advantage for the doublet is economy. A lot of detail can be gathered for a small outlay of dollars with a doublet. Chromatic aberration is usually not a great influence with gemstones as only highlights are refracted in to the differing points of color focus that produce color fringing. Most gemstone observations are in gemstones of one color or colorless gemstones.
Increased color resolution is achieved with a flint glass lens cemented to the inside a double sided convex crown glass lens forming an achromatic compound lens that corrects for the chromatic aberrations that defocus colors in a lens system. This second type of lens system produces straight lines over most of the field of view. An achromatic doublet is a very good, yet still economic 10x system of lenses.
The third lens type, the triplet lens, is a solid system of glass lenses using three layers of glass cemented together forming an aplanatic (in a plane) triplet. The image of a flat plane forms as a plane at the point of focus. This is the best system for hand lenses, when correctly designed and used under normal observation. A triplet has minimal spherical and chromatic aberration and a flat field.
When purchasing a hand lens, buy one that is better than you initially need. If you buy the best triplet lens, use it to your advantage. That s what the whole project should be about. You have to achieve quality observation. There is little advantage in using a quality lens without good lighting and knowledge of how to use both.
When using the hand lens there are several basic rules, and many techniques to develop:
- Spectacles should not be worn unless severe optical defects are present within eyes.
- Choose a good quality light source, with a very narrow beam of high intensity light directed on to your work area and not spilling light towards your eye or the background.
- Work over a soft white cloth that allows easy pick up of gemstones with tweezers. This soft cloth also prevents dropped gemstones from bouncing off the work area on to the floor.
- Clean the sample free from dust and grease (fingerprints) with a lint free cloth. A quality glasses cleaning cloth is suitable.
- Always pick up gemstones with tweezers after cleaning.
- When using the right eye hold the hand lens in the right hand about 25mm in front of the eye, with the index finger primary knuckle resting on the neck.
- Keep both eyes open. This can occur with experience. When the closed eye lid flutters letting light in there is a complimentary iris movement in the open eye.
- Holding the lens close to the eye with one hand, bring the gemstone close with the other hand and rest both hands together for stability. Position the stone 25mm in front of the lens with the gem’s surface in focus, under the light.
- Ensure the lens is straight in front of your eye and is not twisted. Looking off the central axis will produce ‘coma’ color fringing on all images that severely limit quality observing.
- Work systematically around the surface of all faceted gemstones before being drawn into the gems interior. There are many features available for observation, especially around the girdle. Gemstones such as diamond reveal their true nature on the girdle, with natural crystal surfaces left on opposite side of the girdle as the cutters try to achieve maximum weight from the rough stone. The treatment of the girdle or how it was formed is a good guide to the cutting quality and whether care was taken during the gem’s manufacture.
- Do not neglect other features such as flat facets and sharp facet edges that indicate hard gemstones. Concave facets or molding marks at the girdle indicate cast glass. Reflect a fluorescent tube image from the facet for an image of a straight flat facet. Surface luster of facet edges chips indicates the refractive index of the gemstone. Try looking at glass or quartz and comparing surface chips with those on sapphire or diamond. Yes diamond does chip before it cleaves in to two diamonds.
The history of a gemstone’s growth, and subsequent heat and coloring activities induced by man, occurs in the gem’s inclusions. Your 10x hand lens will reveal many of these features under these proposed lighting techniques of use.
Inclusion types have a greater listing than there are gemstone types. Quartz alone has 550 officially listed inclusion types. Quartz is a low temperature forming gemstone; it is of the last to crystallize and includes many other gems that form before it does. Pegmatites in which quartz forms have plenty of liquid and gas so there are many of these inclusions in the ‘veils’ that form within fractures of quartz crystal.
Igneous (volcanic) gemstones can be easily distinguished from metamorphic gems by the individual suites of inclusions within each type. Man-made gems may be chemically identical with those of nature, but man’s techniques of manufacturing these gems create characteristic inclusions that are identifiable with your 10x hand lens.
An interesting feature is found on many older gemstones when a flat plane of air bubbles are glued in a layer between the crown and pavilion of a doublet. These are very easily found with a hand lens. Dispersion of color at individual facets is a good indicator of high refractive index and hardness.
Other hand lenses in use through the industry, such as the Coddington lens and the darkfield loupe have specific features for overcoming specific problems. For example, The Coddington lens is a solid glass cylinder with a restricting light aperture half way at its focal point. This light restriction reduces many aberration errors by preventing them passing the restriction. The darkfield loupe provides correct darkfield lighting for identifying small inclusions in gemstones and direction specific illumination on fracture fill inclusions. This is a small pocket portable, torch based instrument that has power full application in the diamond market of Europe, USA and Asia.
Recommendations
- Use your 10x for it can increase your knowledge and understanding of gemstones.
- A 10x lens is so much faster to use than a microscope that requires more detailed observation. Wear it out as soon as possible and buy a better quality lens as a Christmas present to yourself. If an excuse is needed, it will improve your observing and self-confidence. Do no think that the old 10x will do for the time being.
- Your 10x lens will repay with valuable information.
- Understanding information from a 10x depends on experience gained from informed and correct use f your most important instrument.
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