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The mineral or gemstone chrysoberyl, also known as cymophane or golden beryl, is an aluminate of beryllium with the formula BeAl2O4. Its crystals form in an orthorhombic crystal system with contact and penetration twins common, often repeated forming rosetted structures. Hardness 8.5, refractive index 1.74-1.75, Dispersion 0.015, Double refraction 0.009, optic sign positive, specific gravity 3.5-3.84, distinct cleavage, conchoidal to uneven fracture, vitreous lustre, and colored various shades of green and sometimes yellow. Chrysoberyl is transparent to translucent, sometimes opalescent. The word chrysoberyl is derived from the Greek words meaning golden and beryl.

Chrysoberyl occurs in granitic rockss, pegmatites and mica schists; often it is found in alluvial deposits. It has also been found in contact metamorphic deposits of dolomitic marble with corundum, and in fluorine skarns. Most chrysoberyl is recovered from river sands and gravels.

The alexandrite variety displays pleochroism. Alexandrite from the Ural mountains in Russia is green by daylight and red by transmitted light. Other varieties of alexandrite may be yellowish or pink in daylight and yellow-orange or green by transmitted light. It results from small scale replacement of alumina by chromic oxide, which is responsible for alexandrite's characteristic green to red color change. Alexandrite was first discovered in 1831 in an emerald mining region of the Ural Mountains in the Soviet Union, it is said on the very same day that the Russian czar Alexander II came of age. It was named "alexandrite" in his honor by the mineralogist Adolf Nordenskjold. It is an interesting coincidence that the Russian national colors are green and red.

Translucent yellowish chatoyant chyroberyl is called cymophane or cat's eye. Cymophane has its derivation also from the Greek words meaning wave and appearance, in reference to the opalescence sometimes exhibited. In this variety, microscopic tubelike cavities or needlelike inclusions of rutile occur in an orientation parallel to the c-axis producing a chatoyant effect visible as a single ray of light passing across the crystal. This effect is best seen in gemstones cut in cabochon form perpendicular to the c-axis. The color in yellow chrysoberyl is due to Fe+3 impurities.

Although other minerals such as tourmaline, scapolite, corundum, spinel and quartz can form "cat's eye" stones similar in appearance to cymophane, the jewelry industry designates these stones as "quartz cat's eyes", or "ruby cat's eyes" and only chrysoberyl can be referred to as "cat's eye" with no other designation.

See also: List of minerals