Fine quality alexandrite displays a colour change (alexandrite effect) dependent upon the  light source under which it's viewed. Alexandrite results from small scale replacement of aluminium by chromium oxide, which is responsible for alexandrite's characteristic green to red colour 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" colour change would be fine emerald green to fine purplish red, but this is exceedingly rare.
 00000Because of their rarity and the colour change capability, "ideal" alexandrite gems are some of the most expensive in the world. The are many lesser quality alexandrite on the market, usually pale green and not displaying a noticeable colour change.