Alexandrite

n. type of stone used in jewelry

The mineral or gemstone chrysoberyl, not to be confused with beryl, is an aluminate of beryllium with the formula BeAl2O4. The name chrysoberyl is derived from the Greek words chrysos and beryllos, meaning "golden" and "gem crystal". Despite the similarity of their names, chrysoberyl and beryl are two completely different gemstones. Chrysoberyl is the third hardest naturally occurring gemstone and lies between corundum and topaz on the hardness scale. Chrysoberyl is a mineral consisting of ordinary colorless or yellow transparent chrysoberyl, cymophane (chrysoberyl cat's eye), and alexandrite.
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Noun
1. a green variety of chrysoberyl used as a gemstone
(hypernym) chrysoberyl

n. alexandrite, type of stone used in jewelry

Discovered in 1830 in Russia, and named after Czar Alexander II of who was then Crown Prince of Russia, alexandrite is a form of the mineral chrysoberyl noted for its color change in different forms of light. In sunlight alexandrite looks blue-green, but in indoor (tungsten) light it the same stone changes to reddish-purple. Natural alexandrite with good color is very expensive today, as very little is still being mined, and there are many synthetics on the market. Synthetic color-change sapphire is also sometimes mistaken for alexandrite.