(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.
Wednesday, April 18, 2007
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.
Tuesday, April 17, 2007
Natural Forsterite And Synthetic Forsterite
(via Gemmology Queensland, Vol.4, No.1, January 2003) Hiroshi Kitawaki writes:
Forsterite is one of the end member minerals in the olivine group of minerals. It was named after a British mineral collector Jacob Forster.
Many solid solutions of olivine minerals are known, among which forsterite and fayalite form an isomorphous series. A yellow green crystal, with intermediate composition in this isomorphous series is called peridot. This gemstone is a birthstone of August and is one of the popular gemstones. On the other hand, forsterite is not common a as gemstone.
A forsterite that a chemical formula of, or close to, the end member is rare because one element (Mg) in the formula is generally replaced easily by Fe in nature. The mineral, forsterite, is found in ultrabasic rock or dolomitic limestone that had gone through thermal metamorphism.
Natural Forsterite
The green stone described in this report is a natural forsterite that we recently investigated and it is said to be from Sri Lanka according to its client. Its RI measured 1.635 – 1.670 with DR 0.035. The SG was 3.29 and the stone was inert to UV light. Directionally oriented minute needle-like inclusions were observed under magnification. Its compositional analysis by X-ray fluorescence detected considerable amount of Fe and very small amount of Ca and Mn as well as the main elements of Mg and Si.
Crystals within the olivine group have been extensively studied, and used in heat resistant materials, insulators or lasers. Among the crystals used in industry, single crystal forsterite of high quality and large size have been synthesized by the crystal pulling method for use as crystals that laze the near infrared.
Synthetic forsterite
Synthetic forsterite, which is marketed as Tanzaniod has been synthesized in Russia for gem use. As you can easily imagine from its name, it is meant to imitate tanzanite. The RI, DR and SG of the Tanzaniod are consistent with those of natural forsterite, with Tanzaniod fluorescing a weak orange to yellow and greenish yellow under long and short wave ultraviolet lights respectively. Prominent pleochroism of blue and violet is also recognized. Under the hand-held spectroscope, absorption bands are seen on 490nm, 520nm and 580nm. Dot-like and short needle-like inclusions are observed under magnification. The compositional analysis by fluorescent X-ray detected considerable amount of Co and V, other than the main elements of Mg and Si.
Forsterite is one of the end member minerals in the olivine group of minerals. It was named after a British mineral collector Jacob Forster.
Many solid solutions of olivine minerals are known, among which forsterite and fayalite form an isomorphous series. A yellow green crystal, with intermediate composition in this isomorphous series is called peridot. This gemstone is a birthstone of August and is one of the popular gemstones. On the other hand, forsterite is not common a as gemstone.
A forsterite that a chemical formula of, or close to, the end member is rare because one element (Mg) in the formula is generally replaced easily by Fe in nature. The mineral, forsterite, is found in ultrabasic rock or dolomitic limestone that had gone through thermal metamorphism.
Natural Forsterite
The green stone described in this report is a natural forsterite that we recently investigated and it is said to be from Sri Lanka according to its client. Its RI measured 1.635 – 1.670 with DR 0.035. The SG was 3.29 and the stone was inert to UV light. Directionally oriented minute needle-like inclusions were observed under magnification. Its compositional analysis by X-ray fluorescence detected considerable amount of Fe and very small amount of Ca and Mn as well as the main elements of Mg and Si.
Crystals within the olivine group have been extensively studied, and used in heat resistant materials, insulators or lasers. Among the crystals used in industry, single crystal forsterite of high quality and large size have been synthesized by the crystal pulling method for use as crystals that laze the near infrared.
Synthetic forsterite
Synthetic forsterite, which is marketed as Tanzaniod has been synthesized in Russia for gem use. As you can easily imagine from its name, it is meant to imitate tanzanite. The RI, DR and SG of the Tanzaniod are consistent with those of natural forsterite, with Tanzaniod fluorescing a weak orange to yellow and greenish yellow under long and short wave ultraviolet lights respectively. Prominent pleochroism of blue and violet is also recognized. Under the hand-held spectroscope, absorption bands are seen on 490nm, 520nm and 580nm. Dot-like and short needle-like inclusions are observed under magnification. The compositional analysis by fluorescent X-ray detected considerable amount of Co and V, other than the main elements of Mg and Si.
Natural Hemimorphite And Natural Smithsonite
(via Gemmology Queensland, Vol.4,No.2, February 2003) Hiroshi Kitawaki writes:
We have increasingly encountered natural blue hemimorphite recently. Some of these seem to be easily confused with smithsonite, and this month we are comparing the gemological characteristics of these two stones.
Hemimorphite
The name Hemimorphite was derived from the form of the crystals of this mineral that shows distinct hemimorphism (in which both terminations show different forms). Its Japanese name is Ikyoku-Kou. The mineral is orthorhombic. It is not durable, as its hardness is low at about 4½ to 5 on Mohs scale. However, those hemimorphites with beautiful color, or pronounced transparency, are often cut for jewelry. Colorless, blue, yellow or brown are common, but cabochoned blue stones are more popular these days. The RI of hemimorphite is about 1.61-1.64 with DR 0.022. Its SG is 3.4 to 3.5, and it is inert to UV with no particular feature in the spectrum. Fibrous crystals of hemimorphite characteristically appear striped when examined magnification.
Smithsonite
Smithsonite was named after the mineralogist J Smithson who contributed financially to the establishment of the Smithsonian Institution in Washington, USA. The mineral is named Ryo-Aen-Icou in Japanese, which relates to its chemical composition. Smithsonite is a trigonal mineral, and it is isomorphous with calcite. Although the stone, like hemimorphite, possesses low hardness of 4 to 4½, pose a challenge to its durability. Smithsonites with beautiful colors such as blue, pink, green or yellow will be cabochoned or even faceted. The RI is around 1.62-1.84 and it has a large DR of 0.037. Its SG is 4.3 to 4.5.
When comparing the features of the minerals described above, a SG test will be the most useful way to distinguish them. When use of SG test is restricted due to the presence of setting, elemental analysis or infrared spectral analysis by FTIR will provide you with discriminatory information.
We have increasingly encountered natural blue hemimorphite recently. Some of these seem to be easily confused with smithsonite, and this month we are comparing the gemological characteristics of these two stones.
Hemimorphite
The name Hemimorphite was derived from the form of the crystals of this mineral that shows distinct hemimorphism (in which both terminations show different forms). Its Japanese name is Ikyoku-Kou. The mineral is orthorhombic. It is not durable, as its hardness is low at about 4½ to 5 on Mohs scale. However, those hemimorphites with beautiful color, or pronounced transparency, are often cut for jewelry. Colorless, blue, yellow or brown are common, but cabochoned blue stones are more popular these days. The RI of hemimorphite is about 1.61-1.64 with DR 0.022. Its SG is 3.4 to 3.5, and it is inert to UV with no particular feature in the spectrum. Fibrous crystals of hemimorphite characteristically appear striped when examined magnification.
Smithsonite
Smithsonite was named after the mineralogist J Smithson who contributed financially to the establishment of the Smithsonian Institution in Washington, USA. The mineral is named Ryo-Aen-Icou in Japanese, which relates to its chemical composition. Smithsonite is a trigonal mineral, and it is isomorphous with calcite. Although the stone, like hemimorphite, possesses low hardness of 4 to 4½, pose a challenge to its durability. Smithsonites with beautiful colors such as blue, pink, green or yellow will be cabochoned or even faceted. The RI is around 1.62-1.84 and it has a large DR of 0.037. Its SG is 4.3 to 4.5.
When comparing the features of the minerals described above, a SG test will be the most useful way to distinguish them. When use of SG test is restricted due to the presence of setting, elemental analysis or infrared spectral analysis by FTIR will provide you with discriminatory information.
Natural Hemimorphite And Natural Smithsonite
(via Gemmology Queensland, Vol.4,No.2, February 2003) Hiroshi Kitawaki writes:
We have increasingly encountered natural blue hemimorphite recently. Some of these seem to be easily confused with smithsonite, and this month we are comparing the gemological characteristics of these two stones.
Hemimorphite
The name Hemimorphite was derived from the form of the crystals of this mineral that shows distinct hemimorphism (in which both terminations show different forms). Its Japanese name is Ikyoku-Kou. The mineral is orthorhombic. It is not durable, as its hardness is low at about 4½ to 5 on Mohs scale. However, those hemimorphites with beautiful color, or pronounced transparency, are often cut for jewelry. Colorless, blue, yellow or brown are common, but cabochoned blue stones are more popular these days. The RI of hemimorphite is about 1.61-1.64 with DR 0.022. Its SG is 3.4 to 3.5, and it is inert to UV with no particular feature in the spectrum. Fibrous crystals of hemimorphite characteristically appear striped when examined magnification.
Smithsonite
Smithsonite was named after the mineralogist J Smithson who contributed financially to the establishment of the Smithsonian Institution in Washington, USA. The mineral is named Ryo-Aen-Icou in Japanese, which relates to its chemical composition. Smithsonite is a trigonal mineral, and it is isomorphous with calcite. Although the stone, like hemimorphite, possesses low hardness of 4 to 4½, pose a challenge to its durability. Smithsonites with beautiful colors such as blue, pink, green or yellow will be cabochoned or even faceted. The RI is around 1.62-1.84 and it has a large DR of 0.037. Its SG is 4.3 to 4.5.
When comparing the features of the minerals described above, a SG test will be the most useful way to distinguish them. When use of SG test is restricted due to the presence of setting, elemental analysis or infrared spectral analysis by FTIR will provide you with discriminatory information.
We have increasingly encountered natural blue hemimorphite recently. Some of these seem to be easily confused with smithsonite, and this month we are comparing the gemological characteristics of these two stones.
Hemimorphite
The name Hemimorphite was derived from the form of the crystals of this mineral that shows distinct hemimorphism (in which both terminations show different forms). Its Japanese name is Ikyoku-Kou. The mineral is orthorhombic. It is not durable, as its hardness is low at about 4½ to 5 on Mohs scale. However, those hemimorphites with beautiful color, or pronounced transparency, are often cut for jewelry. Colorless, blue, yellow or brown are common, but cabochoned blue stones are more popular these days. The RI of hemimorphite is about 1.61-1.64 with DR 0.022. Its SG is 3.4 to 3.5, and it is inert to UV with no particular feature in the spectrum. Fibrous crystals of hemimorphite characteristically appear striped when examined magnification.
Smithsonite
Smithsonite was named after the mineralogist J Smithson who contributed financially to the establishment of the Smithsonian Institution in Washington, USA. The mineral is named Ryo-Aen-Icou in Japanese, which relates to its chemical composition. Smithsonite is a trigonal mineral, and it is isomorphous with calcite. Although the stone, like hemimorphite, possesses low hardness of 4 to 4½, pose a challenge to its durability. Smithsonites with beautiful colors such as blue, pink, green or yellow will be cabochoned or even faceted. The RI is around 1.62-1.84 and it has a large DR of 0.037. Its SG is 4.3 to 4.5.
When comparing the features of the minerals described above, a SG test will be the most useful way to distinguish them. When use of SG test is restricted due to the presence of setting, elemental analysis or infrared spectral analysis by FTIR will provide you with discriminatory information.
Subscribe to:
Comments (Atom)