The Mineral chrysoberyl
Chrysoberyl is an uncommon mineral, and the color-changing variety Alexandrite is quite rare and highly valued.
The nature of Alexandrite is very unique. If viewed in sunlight, its color is dark yellow-green to greenish-blue, and if viewed in incandescent light, its color changes strawberry-red to dark pink. This phenomenon is caused by the presence of certain impurities, and only occurs in Chrysoberyl from a few scattered findings.
Chrysoberyl can exhibit a floating light reflection that moves as when a polished gem is rotated. This effect is known as cymophane effect. If a Chrysoberyl exhibits a thin narrow band of reflecting light across its length which slightly moves when a polished gem is rotated, the effect is known as a cat's eye effect. Cat's eye and cymophane effects are caused by microscopic, parallel, needlelike inclusions that cause the light to reflect distinctly within a stone. Both Cymophane and Cat's Eye are variety names used to described Chrysoberyl with these effects. Other minerals and gemstones may also exhibit cymophane or cat's eye effect, but Cymophane and Cat's Eye when used alone are only describing the Chrysoberyl type. For example, Quartz that exhibits cat's eye is known as Cat's Eye Quartz (or Quartz Cat's Eye), whereas Chrysoberyl Cat's Eye is known simply as Cat's Eye.
Alexandrite may also occur in Cat's Eye or Cymophane forms, but this is extremely rare.
For additional information, see the gemstone section on Chrysoberyl, Alexandrite, and Cat's Eye.
Color
Yellow, orange-yellow, yellow-green, dark to light green, brownish-green, bluish-green, brown, gray. The variety Alexandrite becomes reddish in artificial light.
Crystal System
Orthorhombic
Crystal Habits
As thick tabular crystals, as well as long, prismatic crystals. Crystals are often distinctively twinned, either in v-shaped repeated twins or in snowflake-shaped trillings where three individual crystals join to form a unique hexagonal pattern. Crystals are frequently striated, and may be smooth on the edges or rounded due to alluvial action.
Varieties
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Rare variety of Chrysoberyl that exhibits a different color in natural and incandescent light. For more information, see the gemstone section on Alexandrite.
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Gem Chrysoberyl that exhibits a cat's eye effect, in that it reflects a long thin wave of light across a polished cabochon gemstone. Although the term Cat's eye can be applied to other gemstones with this name, the Chrysoberyl form is the most well-known. For more information, see the gemstone section on Cat's Eye.
Uses
Transparent yellow Chrysoberyl is used as a gemstone. The Alexandrite and Cat's Eye varieties are very expensive and desirable gemstones. Transparent yellow and green stones are also used as gemstones, but with much less value and popularity than Alexandrite or Cat's Eye.
Chrysoberyl is also a minor ore of the element beryllium.
Noteworthy Localities
Chrysoberyl is an uncommon mineral, and its occurrences are limited. The original Alexandrite came from the Ural Mountains in Russia, with most of the Russian material coming from the Tokovaya river, Sverdlovsk, in the Ural mountains. Most gem Chrysoberyl today comes from Brazil, specifically Santa Teresa and Colatina in Espirito Santo. Alexandrite from Brazil comes from the Carnaiba Mine, Campo Formoso, Bahia. A recent find of Alexandrite in significant quantity is Masvingo (Fort Victoria), Zimbabwe. Other Chrysoberyl occurrences are Lake Alaotra, Toamasina Province, Madagascar; Rakwana, Sabaragamuwa Province, Sri Lanka; and Ratnapura, Sabaragamuwa Province, Sri Lanka.
In the U.S., Chrysoberyl has been found in the Gillette Quarry, Haddam, Middlesex Co., Connecticut; Topsham, Sagadahoc Co., Maine; and Greenfield, Saratoga Co., New York; and Golden, Jefferson Co., Colorado. Several classic crystals were found in the early 1900's in the West Side of Manhattan in New York City, New York.
Distingushing Similar Minerals
Beryl - Different crystal system, though otherwise different to distinguish.
Topaz - Very hard to distinguish without complex tests, though Chrysoberyl is generally paler in color.