Alexandrite

Alexandrite is a chrysoberyl named after Czar Alexander II and is June's birthstone. It is one of the most desired gems. It is green in daylight and light red in artificial light. This color changes is seen best in thicker stones. Alexandrite cat's eye is a great rarity. Care must be taken when working with it as it is sensitive to knocks, not resistant to alkalies, and the color changes under great heat.
The deposits in the Urals are worked out. Today it is mainly mined in Sri Lanka and Rhodesia. Further deposits are in Burma, Brazil, Malagasy Republic, Tasmania and the U.S. The largest stone of 1876ct was found in Sri Lanka. The largest cut stone weighs 66ct and is in the Smithsonian Institution in Washington. Can be confused with synthetic corundum, which shows a poor green in daylight, but a good strawberry red under artificial light. There are also doublets with a good color change on the market; red garnet on top, red glass underneath.

Chrysoberyl:

Color: Golden yellow, green-yellow, brown
Color of streak: white
Mohs' hardness: 8 1/2
Specific Gravity: 3.70-3.72
Cleavage: Imperfect
Fracture: Weak; conchoidal
Crystal system: Orthorhombic; long prismatic crystals intergrown triplets
Chemical composition: Al2(BeO4) beryllium aluminum oxide
Transparency: Transparent
Refractive index: 1.744-1.755
Double refraction: +0.011
Dispersion: 0.015
Pleochroism: Very weak; reddish-yellow, yellow, light green, green
Absorbtion spectrum: 5040, 4950, 4850, 4450
Florescence Usually none, Green : Weak ; dark red

Chrysoberyl (Greek - gold) has been known since very early times. Today the varieties alexandrite and chrysoberyl cat's eye are especially valued.

* Passage taken from Gemstones of the World by Walter Schumann

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