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Emeralds vary in color from light to deep green. It's commonly thought that an emerald's color derives from the presence of chromium and/or vanadium, replacing some of the aluminum in the mineral's structure. The stone can, however, lose its color when heated strongly.

The emerald belongs to the beryl family of minerals that include aquamarine (the March birthstone), heliodor and morganite. Beryl, or beryllium aluminum silicate in chemical jargon, is a six-sided symmetrical crystal. Beryl contains beryllium, aluminum, silicon and oxygen.

Emeralds are most frequently found inside a form of shale -- a fine grained sedimentary rock. Emerald-bearing shale has undergone recrystallization due to changes in the physical environment such as pressure and temperature. Colombia produces the largest and highest quality emeralds. They were also discovered, and subsequently mined, in the Ural Mountains of Russia around 1830. In the United States, emeralds can be found in North Carolina. Around the world, they also occur in Zambia, Brazil, Pakistan, Norway, Austria, India, Malagasy and Australia.

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Emerald is a variety of the species beryl. Its chemical composition is a combination of beryllium aluminum silicate and it appears as a colorless crystal in its pure state. Trace elements present in the chemical mix cause the colors of the various beryl varieties. The elements that can cause the green in beryl are chromium, vanadium or iron. In order to be considered a true emerald, the Gemological Institute of America states that a beryl crystal must be colored green by the element chromium or vanadium or both.

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http://webmineral.com/data/Beryl.shtml

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