Gems : Chrysoberyl

Chrysoberyl is not one of the best known gemstones, although its rarest variety, alexandrite, is quite well known. Chrysoberyl is harder than topaz, and also includes a variety known as cat's eye.
Most chrysoberyl is green or yellow-green, but some is brown. Alexandrite shows a colour change from green to red, and cat's eye is usually green or yellow-green, with a strongly banded appearance well described by its name.