A chrysoberyl gemstone featuring a somewhat asymmetric, "native" oval mixed cut.

Some gemstones described as lab-grown (synthetic) alexandrite are actually corundum laced with trace elements (e.g., vanadium) or color-change spinel and are not actually chrysoberyl. As a result, they would be more accurately described as simulated alexandrite rather than synthetic.
Synthetic alexandrite is used as an active laser medium. Alexandrite laser crystals tend to be round, with a pale brown tint

Cymophane variety

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 chatoyancy 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 Fe3+ 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.

References
- Klein, Cornelis, and Cornelius S. Hurlbut, Jr. (1985). Manual of Mineralogy, 20th ed., New York: Wiley. ISBN 0-471-80580-7
- Weinstein, Michael (1958). The World of Jewel Stones. New York: Sheridan House.

Source from Wikipedia