My absolutely favorite gem is Aleksandrite. It is a variety of Chrysoberyl. Aleksandrite was discovered in the Malyshevo emerald district of the Srednyi Ural. It has the very peculiar property that it changes color. When viewed in daylight the Aleksandrite is green like emerald and when viewed in candle light it is red like ruby. Because red and green where the colors of the Russian flag in those day's it soon became Russia's national stone. It was named after Tsar Aleksander.
I tried my best to capture the color change in pictures, but it still does not reflect the real effect. At least you have got an idea,
[photo: Maurice de Graaf, collection Ingrid Weikamp]