Emerald is a variety of the mineral beryl, colored green by trace amounts of chromium and sometimes. Beryl has a hardness of 7.5 - 8 on the 10 point Mohs scale of hardness. Most emeralds are highly included, so the brittleness (resistance to breakage) is classified as generally poor. The origin of the word "Emerald" is said to be a Sanskrit word meaning "green".
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. The value of an emerald depends on cut, color, clarity, and carat. At this point in time the best emeralds come from the Muzo mine in Colombia.
Emeralds in antiquity were mined by the Egyptians and in Austria, as well as Swat in northern Pakistan. A rare type of emerald known as a trapiche emerald is occasionally found in the mines of Colombia. A trapiche emerald exhibits a "star" pattern; it has raylike spokes of dark carbon impurities that give the emerald a six-pointed radial pattern. It is named for the trapiche, a grinding wheel used to process sugarcane in the region. Colombian emeralds are generally the most prized due to their transparency and fire. Some of the most rare emeralds come from three main emerald mining areas in Colombia: Muzo, Coscuez, and Chivor. Fine emeralds are also found in other countries, such as Zambia, Brazil, Zimbabwe, Madagascar, Pakistan, India, Afghanistan and Russia. In the US, it can be found in North Carolina.
Treatments
Most emeralds are oiled as part of the post lapidary process, in order to improve their clarity. Cedar oil is often used, having a similar refractive index, and it is a generally accepted practice. The U.S. Federal Trade Commission requires the disclosure of this type of treatment when a treated emerald is sold. The amount of oil entering an emerald microfissure is roughly equivalent to the size of a period in print. The use of green tinted oil is generally not considered acceptable by the gem trade.