Alexendrite, named after Czar Alexander II, is famous for its ability to change color with change in the intensity of light, from bluish mossy green in strong light to red in dim room light. You can watch it flick back and forth by switching from fluorescent to incandescent light.
Since the original mines in Russia have closed down so to find a fresh Alexandrite stone is very difficult. Material with a certificate of Russian origin is still particularly valued by the trade. Some alexandrite is found in Sri Lanka and Zimbabwe and Brazil but very little shows a dramatic color change. However, crystals of synthetically grown Alexandrites have also gained importance in the market.
Value of the stone is generally gauged by the amount of change in color that it produces. The ones with more dramatic color change are more valuable and hence, more expensive. They are available in mm sizes and far sizes (Usually Below 5 Carats) in shape of Round, Oval, Pear, Heart, Marquies, Trillion, Square, Octogun, Cushion etc.