GemSelect Newsletter - November 2009
Gemstones with unusual optical qualities have held a special fascination for centuries. In gemology these are known as phenomenal gems. These effects include chatoyancy (the cat's eye effect), asterism (the star effect), adularescence (the shimmering effect, as in moonstone), play of color (as in opal) and iridescence (rainbow-like colors, as in labradorite).
More recently there is another optical effect that has drawn a lot of attention - color change. Color change gemstones display different colors according to changes in lighting. The most famous of these is the rare and expensive alexandrite, a variety of chrysoberyl. However, color-change garnets and color-change sapphires have appeared on the market and show equally dramatic color changes.
Some quite stunning color-change garnets have been discovered in Madagascar and East Africa in the last few years. Some of this material shows a marked change from greenish-brown to red-pink; others change color in various shades of orange or pink. We have even seen some pieces show a bluish-purple to red-pink change. We have been buying as many of the color-change garnet gemstones as we can find, since supply in these rare stones tends to be short-lived.
A more subtle color change is found in the lovely pastel-colored diaspore. Mined from a single deposit in the mountains of Central Turkey, it is sometimes sold under the marketing name zultanite or csarite. Under natural or fluorescent light, diapsore has a kiwi green color, with flashes of yellow. Diaspore displays a champagne color under incandescent lighting, and when exposed to subdued lighting, such as candlelight, it has a pinkish color.
Diaspore has reasonably good gemstone characteristics, with a hardness of 6.5 to 7 on the Mohs scale, about the same as peridot and tanzanite. It also has very good brilliance, with a high refractive index in the range between tanzanite and spinel.
Gemologists have discovered some of the secrets of color-change gems. Apparently some gemstones have two, approximately equal-sized transmission windows. A red gemstone appears red because it absorbs all frequencies of light except for red. A gemstone that absorbs all frequencies except for blue and red light will appear blue when the light is rich in blue wavelengths (e.g., fluorescent light) and red when the light is rich in red wavelengths (e.g., incandescent lighting). The dual personality of color-change gems makes them fascinating to observe under changing conditions.
Each month we focus on a rare and unusual gem from our inventory. This month we feature an exceptional, large opal gemstone:
We often stock small gemstone carvings, but recently we've acquired some unusual large carvings produced by local artisans here in Thailand. These unique pieces, carved from a single block of material, are treasured by collectors. Our latest acquisition is this 670 carat carving of two iguanas. Executed with remarkable detail in Australian opal, this carving is almost 110 mm (4.33 inches) tall and would make a wonderful collector's piece or decorative gemstone ornament.
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Happy Gem Hunting!
Your friends at GemSelect
- First Published: November-01-2009
- Last Updated: June-29-2017
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