By Arthur Skuratowicz, G.J.G. (Graduate Jeweler Gemologist) (Text by
Nancy L. Attaway)
Arthur Skuratowicz spoke to the Guild on the problems encountered during gemstone identification. Arthur, a GIA Graduate Jeweler Gemologist, taught metalsmithing, gemstone identification, and faceting at GIA for seven years. Arthur is a master jeweler and appraiser at Sheltonís Jewelry.
The terminology and the instruments used in gemstone identification provide clues for the detective work that determine the identity of a gemstone, but it is the actual thought process of how these clues are utilized that enables a final answer. Terminology allows a means to describe in words what we are seeing in a particular way that other people will understand. The clues leading to the identity of a gemstone can be written on a chart and compared with data known for specific gemstones.
Gemologists use different instruments for gathering data during the
gemstone identification process. The most useful of these is the microscope,
especially a stereoscopic type fitted with special darkfield illumination
that permits light to enter the gemstone only from the sides. A microscope
serves as the ideal tool for examining inclusions that can provide positive
proof in many cases of a gemstoneís identity. Magnification also reveals
internal features of a gemstoneís crystal structure. The most commonly
used tool is the small 10X loupe. A loupe allows a close, but limited,
inspection of a gemstone, both inside and out, while being very portable.
Several other instruments play important parts in the gemstone identification process. The refractometer is a tool used to measure the refractive index and the birefringence of most gemstones. Gemstones bend light at measurable angles, and this data can be compared to a listing that shows the refractive index readings known for specific gemstones. A gemstone that breaks one beam of light into two polarized beams is said to be doubly refractive, having a birefringence, and this gives yet another clue. The polariscope is a tool used to determine the optical character of a gemstone, and it shows whether the stone is singly or doubly refractive.
The spectroscope is another tool that separates white light into a spectrum of component colors with a prism or a diffraction grating, where each slit represents a different wavelength. A gemstone placed between the light source and the slit will absorb certain wavelengths that can be measured to yield another clue. A dichroscope is a small tool that determines pleochroic colors, giving a clue to the crystal system of a gemstone. All these tools provide very useful information for the gemologist. It is important to control the lighting when performing these tests.
Gem buyers can and do make mistakes when purchasing colored gemstones. The more knowledge and experience are acquired, the less mistakes a buyer makes. It becomes important to be familiar with many types of gemstones. We need to view actual examples first-hand.
Many gemstones exhibit telltale or signature appearances. Observe the obvious. Note the crystal habit, the overall color, and the color distribution. Then, look inside. A gemstone may contain inclusions that can provide a key to its identity. Rubies, sapphires, emeralds, and quartz are examples of gemstones that show natural inclusions of how the crystal grew, marked by fingerprint patterns and zonal banding in straight lines. Curved lines could indicate synthetic origin. A gemstone that contains an inclusion of a distinct natural crystal, like calcite in emerald or apatite in ruby, would be an indication that the host gemstone is of a natural variety.
Some inclusions show evidence of heat-treatment. In some cases, heat can cause fractures to heal, occasionally seen in rubies and sapphires. Sometimes the temperatures in heat-treatment become high enough to cause the inclusions to expand, generating fissures inside the gemstone. In the case of ruby, very often these fissures are glass-filled when they reach the surface. The presence of platinum inclusions reveals a gemstone to be synthetic, as platinum crucibles are used in growing gemstones. The PhotoAtlas of Inclusions by Gubelin and Koivula is a highly recommended reference book that contains marvelous pictures of many types of inclusions. Comparing the inclusions in gemstones to those in the PhotoAtlas of Inclusions can provide keys to a gemstoneís identity. Colorless gemstones seem to be somewhat more difficult to determine. The same can also be said of the opaque gemstones cut en cabochon.
Some gemstones need to be heat-treated to improve and/or enhance their color. Tanzanite is heat-treated to improve the overall color, and this treatment is permanent. Sapphire is heat-treated to both improve the color tones and enhance the clarity by melting the rutile, and this is also a stable treatment. Diffusion-treated sapphires usually show color concentrations close to the surface, and this color may disappear when the stone is re-cut. Irradiated sapphire changed to red will fade to yellow in ultraviolet light. The natural color in kunzite will fade with exposure to ultraviolet light, as will morganite. The deep blue color of Maxixe beryl quickly fades in sunlight. Blue topaz is both natural and heat-treated, as well as irradiated, but heat-treatment in blue topaz cannot be proved. Aquamarine is sometimes, but not always, heat-treated to remove the yellow, thus changing the color from light bluish green to blue. Heat-treated amethyst becomes deep yellow citrine. The color that we expect to see in gemstones is usually yielded by heat-treatment.
Diamonds are irradiated to improve or change the color, and these colors remain stable. Diamonds are irradiated inside a cyclotron or in a linear accelerator and later annealed to set the color. In the case of green diamonds, tests cannot prove exactly when the irradiation occurred if green irradiation stains are not present on a facet.
Be aware of fraudulent treatments now found in colored gemstones. Tigereye, agate, pearls, lapis and jadeite can all be dyed to enhance the color. Look for dye concentrations in the matrix. The calcite absorbs the dye in the matrix of lapis. Dye concentrations in pearls may be spotted at the drill holes. Pearls are also soaked in salt solutions, bleached, tumbled to improve roundness, and sometimes irradiated. The luster and the thickness of the nacre on freshwater and saltwater pearls are dependent upon the temperature of the water and the specie of the mollusk used. Pearls can be x-rayed to see the layers and the seed, but the interpretation of the x-rays is not easy. Turquoise is subjected to a wide variety of treatments, and many are not easily discovered through testing.
Emeralds filled with clear oils and waxes to enhance clarity is accepted as normal, but these are not stable treatments. Emeralds filled with dyed waxes and dyed oils are definitely fraudulent. Some synthetics and imitations made to mimic emeralds are quench-cracked and filled with green oil, again fraudulent. Gems with a crystalline emerald coating have been revealed. The use of Opticon and other polymers and resins are still in question. A treatment not disclosed is now considered fraud.
Laser-drilling of diamonds and fracture-filling of rubies not disclosed is also fraudulent. Laser-drilling improves the apparent clarity of a diamond and raises its saleability. However, it does not increase the clarity or the value, as a dark inclusion is traded for a hole in the diamond. When a jeweler unknowingly heats a fracture-filled diamond during a repair, the diamond will be damaged. A fracture-filled diamond may exhibit a characteristic flash of color. Treaters argue that fracture-filling of rubies occurs unintentionally during heat-treatment, where borax seeps into the cracks. The fracture-filling of rubies is also done intentionally, and it is not always disclosed.
Opal can be subjected to a variety of treatments. Opal can be soaked in a sugar solution and then dipped in sulfuric acid to darken the color. Such treated opal reveals a peppery black appearance under magnification. Smoke and ashes are also used to darken opal. This treatment shows a flaky surface under magnification. Oils, waxes, and polymers can be injected into opals with heat and pressure, and these are hard to detect. Be familiar with the appearance of crystal opal, black opal, and Mexican opal. Thin layers of opal are also made into doublets and triplets, and these should always be disclosed.
The color-change gemstones include alexandrite, sapphire, and garnet (spessartine/almandine from East Africa). These unusual gemstones change colors when placed in incandescent and then taken to ultraviolet light.
Several gemstones are difficult to determine. Some heat-treated sapphires
have tested natural. Synthetic amethyst is now very hard to determine.
When amethyst was first grown in a lab, the seed crystals were untwinned.
Twinning, then, became a mark of natural amethyst. Amethyst can now be
grown twinned, and it is a challenge to determine synthetic from natural.
Flux-grown synthetic spinel is another gemstone very hard to identify.
Faceted stones of this variety test natural because most of the few inclusions
it ever had, left from the growth process, have been removed from cutting.
Some of the inclusions found in nature are now copied in man-made gemstone material. For example, Columbian emeralds grow naturally in a hydrothermal environment. Lab-grown emeralds are grown by man hydrothermally. Likewise, the nailhead spicules found in hydrothermal lab-grown emerald mimic the shapes of phenakite sometimes seen in natural beryl.
The first man-made diamond appeared in 1954, but it was industrial grade. The first man-made diamond of gem quality became a reality in 1971 by cooking carbon under pressure. Nitrogen can allegedly be leached from a industrial grade diamonds to improve the color, fading the yellow tones to white. Hypothetically, if this method can be applied to gem quality diamonds, then it may raise the apparent value of such a diamond, if the treatment is not detected. The color is stable, but the treatment may not be detectable.
The introduction of synthetic moissanite to the gemstone market has upset many jewelers, because synthetic moissanite tests like a diamond. A telltale clue for identifying synthetic moissanite is that it is doubly refractive. Knowing this, cutters of synthetic moissanite are now orienting the table down the axis of the stone to diminish this characteristic. Tilt the stone to one side and look through the crown facets to reveal the double refraction.
Gemstones exhibit a variety of phenomenon, and these can serve as clues to identification. The gemstones that display a six-rayed star include sapphire, aquamarine, and quartz. Garnets show a four-rayed star because they represent a different crystal system. However, garnets can show a six-rayed star if the two crossing needles are oriented with the needle intersecting them perpendicularly. The trapiche emeralds, unique to Columbia, exhibit six sections of green when cut down the C axis. Sometimes, these are divided by six black lines like spokes in a wheel. The gemstones that exhibit a natural catís-eye include alexandrite, emerald, aquamarine, rose quartz, opal, and chrysoberyl. Some imitation catís-eye stones are actually fiber-optic glass, and these reveal hexagonal patterns inside of hexagonal patterns under high magnification. Gas bubble striations are seen in synthetic catís-eye alexandrite. Amber and the natural glasses (obsidian) are the only gemstones that contain a free-standing gas bubble. When chipped, devitreous glass is granular.
A gemologist depends upon observational skills. Practical gemology has
no specified order of instrument usage. However, research gemology relies
on a specific and conclusive sequence of instrument usage. A gemologist
incorporates an acquired knowledge of gemstones with the usage of many
different laboratory instruments to obtain clues that yield the identity
of a gemstone. The identification process becomes more difficult with the
introduction of synthetics, imitations, and treatments that prove hard
to detect. A gemologist needs to keep aware of new methods of crystal growth
and gemstone treatments to guard against mis-identification and fraud.