Tanzanite, Iolite, Chrysoberyl, and Andalusite PDF

Tanzanite, Iolite, Chrysoberyl, and Andalusite KEY CONCEPTS Pleochroic and phenomenal gems must be precisely oriented Alexandrite chrysoberyl is the finest color-change gem, and to display their optical effec...

Tanzanite, Iolite, Chrysoberyl, and Andalusite KEY CONCEPTS Pleochroic and phenomenal gems must be precisely oriented Alexandrite chrysoberyl is the finest color-change gem, and to display their optical effects properly. the standard against which all other color-change gems are judged. Tanzanite’s success in the marketplace was due in part to enthusiastic promotion. Traces of chromium cause alexandrite’s color change. The best tanzanite color is a strongly saturated pure blue or Fine alexandrite is green to bluish green in daylight or violetish blue. fluorescent light and red to purplish red in incandescent light, with medium to medium-dark tone and moderately strong Due to tanzanite’s strong pleochroism, cutters must balance saturation. weight retention against quality of face-up color. Although Russian alexandrite mines produce little today, the Tanzanite is almost always heat-treated to bring out its more rare and vivid hues of Russian alexandrite still set the standard attractive blue and violet hues. for fine color. Tanzania is the only commercial source of gem-quality A large deposit of alexandrite discovered in Brazil in the early tanzanite. 1980s briefly increased alexandrite’s availability. Because tanzanite comes from a single source, changes in Currently, fine alexandrite is so scarce that it has virtually supply drastically affect availability and prices. disappeared from the gem marketplace except as an expensive collector’s gem. When moderately priced commercial-quality blue sapphire is available, it’s more marketable than commercial-quality Excellent hardness and durability combined with the sharpest tanzanite. cat’s-eye make chrysoberyl the most-prized chatoyant gem. Synthetic forsterite is the most convincing tanzanite imitation Cat’s-eye chrysoberyl’s chatoyancy is caused by light reflecting to date. from fine needle-like inclusions within the gemstone. Iolites with a predominantly blue face-up color are most Because it withstands rough wear and is subtly colored, cat’s- valuable, so cutters orient the finished stones to display the eye is traditionally marketed as a man’s gem. desirable color. Andalusite’s pleochroism is so pronounced that its hues are Iolite can be an affordable alternative to tanzanite because in often visible simultaneously through the gem’s crown, creating comparable quality, it sells for about one-tenth the price. a mosaic effect. 327