Emeralds are a variety of the mineral beryl (Be3Al2(SiO3)6,) colored green by trace amounts of chromium and sometimes vanadium. Hurlbut, Cornelius S. Jr, & Kammerling, Robert C., 1991, Gemology, p. 203, John Wiley & Sons, New York. Beryl has a hardness of 7.5 - 8 on the 10 point Mohs scale of mineral hardness. Hurlbut, Cornelius S. Jr, & Kammerling, Robert C., 1991, Gemology, p. 202, John Wiley & Sons, New York Most emeralds are highly included, so their brittleness (resistance to breakage) is classified as generally poor. The origin of the word "emerald" is said to be a Sanskrit word meaning "green".
Properties determining value
Emeralds come in many shades of green and bluish green. There is a wide spectrum of clarity, dependent on the inclusions and fractures in the crystal. Clear stones with dark yet vibrant color command the highest prices. Almost all emeralds contain numerous flaws, cracks, and inclusions, which can negatively affect the clarity. These are given the name "
Source:
http://en.wikipedia.org/wiki/Emerald