Alexandrite

Alexandrite is an extremely rare color-change gem.

It is a phenomenal variety of chrysoberyl that changes from greenish in daylight to reddish in lamplight.  Alexandrite is sometimes described as "emerald by day, ruby by night".

Alexandrite was originally discovered in Russia's Ural Mountains in the 1830s and was named after the Russian tsar, Alexander II.  It is now also found in Sri Lanka, East Africa, and Brazil.