Alexandrite and other varieties of Chrysoberyl

Alexandrite and other varieties of Chrysoberyl
The mineral or gemstone chrysoberyl, not to be confused with beryl, is an aluminate of beryllium with the formula BeAl2O4. Chrysoberyl is transparent to translucent and sometimes chatoyant. An interesting feature of uncut crystals of chyrsoberyl are the cyclic twins called trillings. These twinned crystals have a hexagonal appearance, but are the result of a triplet of twins with each "twin" taking up 120 degrees of the cyclic trilling. The word chrysoberyl is derived from the Greek chrysos, "golden," and beryllos, of uncertain etymology.

Occurrence
Chrysoberyl occurs in granitic rocks, 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 bearing skarns. Most chrysoberyl is recovered from river sands and gravels

Alexandrite variety
The alexandrite variety displays a color change (alexandrite effect) dependent upon light, along with strong 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. Because of their rarity and the color change capability, "ideal" alexandrite gems are some of the most expensive in the world.
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 were found in the Ural Mountains, 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.), India (Andhra Pradesh), 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.

Source from Wikipedia