Alexandrite
June Birthstone

Raw alexandrite from the Urals
 

Scientific:
Alexandrite owes its name to the fact that it was discovered in the Urals in 1830, on the day that Prince Alexander of Russia's coming of age (turning 21).

Alexandrite is a very rare mineral variety of chrysoberyl, which occurs in granitic pegmatites and mica schists. Small-scale replacement in chrysoberyl of alumina by chromic oxide results in the formation of the variety alexandrite and is responsible for alexandrite's characteristic green to red color change. Poets have spoken of alexandrite as "an emerald by day and a ruby by night."

It is green in daylight and red in artificial incandescent light. It can be almost emerald green, but is more often yellowish or brownish green. It's main characteristic is the ability to change color if exposed to a light source rich in red rays; by candlelight or tungsten light, it turns red or reddish. This unusual phenomenon is what distinguishes it from other green chrysoberyl. This pronounced color change from green to red is highly prized and, as always, the exact tone of the color, or colors, is important. The ideal color change is brilliant green turning into fiery red, although dull green turning reddish or slightly turbid blood red is more common. This changing color is best seen in thick stones. Alexandrite from the Urals is blue-green in daylight and bright purplish-red under tungsten light. The stone from Ceylon is olive-green to brown in the day light and red under artificial light.

Given the rarity of alexandrite, even stones with a number of inclusions are cut. Alexandrite displaying the cat's eye effect is a great rarity. 

The changing color is an unmistakable characteristic. However, recently, small quantities of green or bluish colored grossular garnet that turn red like alexandrite have been found in South Africa. To distinguish between the two, it is useful to look for signs of birefringence, as garnet, unlike alexandrite, is singly refractive. (Birefringence is the ability of certain crystals to split incident light into two rays.)

Alexandrite is so rare that very few people have actually seen one; and perhaps for this reason, the name is applied to other, mainly synthetic stones, not remotely like it (usually violet colored synthetic corundum's). 

Care must be taken when working with it, as it is sensitive to knocks and color changes are possible with exposure to great heat. High quality alexandrite is one of the most expensive of all gemstones.

The deposits in the Urals are all worked out. Today it is mined in Sri Lanka, Zimbabwe, Brazil, Burma, Madagascar, and Tanzania. The largest stone, 1876 ct, was found in Sri Lanka. The largest cut alexandrite weighs 66 ct; it is in the Smithsonian Institute in Washington, D.C.

Alexandrite is used only in jewelry. It is so rare that anything over 5 carats is considered big!

Variety of: Chrysoberyl
Chemical Composition: BeAl2O4 + Fe,Ti beryllium aluminum oxide
Hardness: 8.5