Alexandrite

Alexandrite is technically called Chrysoberyl and is an aluminate of the chemical element beryllium.
Named after the Russian Tsar Alexander II, and was discovered in 1834 in Russia and is a very rare mineral which exhibits a fascinating colour change in different lighting, being green or bluish green in daylight and turning a soft shade of red or rasberry red in incandescent light. This characteristic along with it's scarcity make it one of the most valuable gemstones of all.
Alexandrite is the third hardest known gemstone with a hardness of 8.5 on the Mohs scale.

  

Some examples of Alexandrite seen in different light - Daylight version on the left, other lighting on the right