The alexandrite variety displays pleochroism. Alexandrite results from small scale replacement of aluminium by chromium oxide, which is responsible for alexandrite's characteristic green to red color change. Alexandrite from the Ural mountains in Russia is green by daylight and red by incandescent light. Other varieties of alexandrite may be yellowish or pink in daylight and a columbine or raspberry red by incandescent light. The optimum or "ideal" color change would be fine emerald green to fine purplish red, but this is exceedingly rare.

Alexandrite was first discovered in 1831 in an emerald mining region of the Ural Mountains in Russia. The name comes from Tsar Alexander II of Russia, on whose birthday the gem was discovered in that country. It was named "alexandrite" in his honor by the mineralogist Adolf Erik Nordenskiöld. It is an interesting coincidence that the Russian national colors are green and red, which may have originated from this gem.

The finest alexandrites are still found in the Ural Mountains of Russia, the largest cut stones being in the 30 carat (6 g) 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 (200 mg) but with intense color change). Overall, stones from any locale over 5 carats (1 g) 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-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.

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