Alexandrite is a variety of chrysoberyl. It changes its colour according to the light: in natural or fluourescent light it is generally olive green, but in incandescent light, a columbine or raspberry red. The optimum or "ideal" color change would be fine emerald green to fine purplish red, but this is exceedingly rare. Most synthetic versions (actually corundum, i.e. sapphire, which shows a notable color change) are usually easily identifiable due to their light green to medium purple color change.
It is associated with the astrological sign of Gemini and the month of June. Alexandrite is an emblem of loyal regard. Dreams of alexandrites are said to symbolize struggle and progress.
The name comes from Tsar Alexander II of Russia, on whose birthday the gem was discovered in that country. The finest alexandrites are still found in the Ural Mountains of Russia (the largest cut stones being in the 30 carat range), though many fine examples have been discovered in Sri Lanka (up to 65 cts.), Brazil, Myanmar, and especially Zimbabwe (small stones usually under 1 carat but with intense color change). Overall, stones from any locale over 5 carats would be considered extremely rare, especially gems with fine color change. Alexandrite is both hard and tough, making it very well suited to wear in jewelry.
The gem has given rise to the adjective "alexandritic", meaning any transparent gem or material which shows a noted change in color between natural and incandescent light. Some other gem varieties of which alexandritic specimens have been found include sapphire, garnet, and spinel.
Some gemstones described as lab alexandrite are actually courundum laced with trace elements and are not actually chrysoberyl. As a result, they would be more accurately described as simulated alexandrite rather than synthetic.
Chemical composition - Al2(BeO4) beryllium aluminium oxide
Hardness - 8.5 mohs
See also: list of minerals