All That Glitters: Precious Gems
From the Print Edition:
Wayne Gretzky, Mar/Apr 97
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Fancy colored sapphires are rated according to the intensity of their hue: to be valuable, they should be vivid in color, and very clean and clear. Choice of color is strictly in the eye of the beholder: if you like green, or yellow, or violet, these are the stones that will catch your eye.
However, if emerald green is the color that you crave, then only an emerald will do. Of the big three colored gemstones, emeralds occupy their own category. Not nearly as hard as ruby and sapphire, emerald is a form of beryl and is rated eight on the hardness scale. The color derives from small amounts of chromium, enhanced by traces of iron. Don't attempt to find a truly clean emerald: these stones nearly always have deep inclusions, that is, tiny imperfections in the form of crystal structures within the stones. These inclusions have been given beautiful names such as "silk" and "jardin," meaning garden. Emerald's beautiful color forces people to overlook these flaws; they never bothered the Moguls of India, who loved their big emeralds no matter how included they were.
Historically, the richest source of emeralds has been South America. The Muzo mine, about 160 miles north of Bogota, Colombia, has been producing for thousands of years, give or take a few insurrections. "There is a unique geology in the mountains there," Cheryl Kremkow says. "The existing mines are along one formation. They have been blessed with such incredibly rich deposits no one has looked for more." Emerald also has its mystical sources: Sandawana emeralds, from Zimbabwe in Africa, have always found ready buyers who value them for their beauty, though they generally occur only in small sizes. Among the newer sources of emerald is Zambia, in central Africa. Zambia emerald quickly became one of the most important sources, ranking below that of Colombia for value. The deposit occurs near the copper mines, a common occurrence. These stones, as Nassi said, have had to struggle to make their way in the world simply because they originate from a new source, not from some long-established, highly esteemed mine. It would appear that newness is simply suspect in the highly secretive, closed world of gems and gem dealers.
New York City gem dealer Doug Parker of Wm. Kuhn Co. echoes Nassi's displeasure regarding gem origins. "I like beautiful stones; I buy for beauty, and I dislike the trade's emphasis on origin. I buy a lot of Zambia material; I can get clearer stones. The crystalline structure of the Zambian stones seems to be a little stronger."
The love of emerald green is so strong it overcomes emerald's inherent drawback: the fractures create weaknesses in the stones that make them susceptible to cracking. If the fracture is on the surface, emeralds are best worn as earrings or necklaces, rather than as rings or bracelets where they are far more subject to contact with hard surfaces. For the goldsmith, setting emeralds is a far more perilous task than the setting of the far tougher rubies and sapphires. The characteristic shape of emerald was the source of inspiration for the emerald cut, a simple rectangle faceted with "steps."
Emeralds are usually helped along in other ways as well. Most emeralds are oiled, which fills in the cracks and helps to stabilize the stones. This is a way of vastly improving the quality of a given stone. Such treatments are widely practiced, and are the reason that an emerald requires the most delicate of care. Emerald jewelry should not be cleaned in an ultrasonic cleaner, because the ultrasonic waves would prey on the weakness of the fractures in the stone and crack the stone into pieces, and the oil could be removed in the process, leaving you with a more fragile stone.
But emeralds are scarcely the only stones to undergo treatment. Blue sapphires are routinely heat-treated to intensify their color. Sometimes the original gem material is actually colorless, but through the application of heat, a beautiful blue sapphire may emerge. Sri Lankans are masters at this, though suspicions were raised only because they were producing so much material of such excellent color. One of the drawbacks of heat treatment is that it tends to mask the inclusions that give clues to the origin of the stone.
The treatment is permanent--a heat-treated blue sapphire will not revert to its original color, or colorless state. The practice has become so prevalent that it is now more reasonable to assume that a sapphire has been treated than not. Only the finest, most important stones are being subjected to laboratory examination to determine and certify that a stone has not been heat-treated. This is only practical if the stone is of great value. Small stones used in jewelry or run-of-the-mill stones of modest value do not justify certification. Heat treatment is as old as the hills in which the stones are found. The technique was being discussed in the sixteenth century by the brilliant goldsmith Benvenuto Cellini.
There is another drawback to heat treatment, however. Some sapphires, and some rubies, exhibit a special internal structure that results in asterism--there appears to be a star within the stone. These stars are actually rutiles, needle-like mineral inclusions. But heat treatment absorbs the rutiles, so while the color may be improved, the asterism is lost.
Unlike diamonds, where clarity is of prime importance, in colored gemstones the operative word is color. Cut takes a distant backseat in colored stones because their beauty does not depend on reflected light. Most are cut simply, to take advantage of the shape of the rough. There is no ideal cut, as there is in diamonds. Since many colored gems are unevenly colored in nature, the cutter's job is to take advantage of the color present and create stones with uniform appearance.
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