Emerald is the green variety of the mineral beryl. Other gemstone color varieties that belong to beryl include aquamarine, morganite, heliodor, and goshenite. Other colors of beryl are simply referred to by their color, such as red beryl.
The wonderful green color of emerald is unparalleled in the gem kingdom. Emerald's precious green color is caused by small amounts of chromium and enhanced by traces of iron. Unlike other beryls, emeralds often contain inclusions and other flaws. These flaws are not looked on as negative aspects for emerald like they would be for other gemstones. Indeed, these flaws are considered part of the character of the stone and are used to assure the purchaser of a natural stone. However, fractures and inclusions are so common in emeralds that their toughness is affected - emeralds tend to break more easily than other beryls.
Nearly all emerald gemstones have been treated to improve their appearance. Historically, gems were "oiled" which filled the cracks and veils with a transparent oil, effectively hiding the flaw. Unfortunately, oils tend to evaporate over the years (and they may be removed with detergents) so that oil-treated emeralds tend to look worse over time. Luckily, the treatment may be repeated to good effect. In recent times, the cracks are filled with a permanent epoxy so evaporation of oil is not an issue.
In any case, you should expect an emerald to be treated. Note that oil (or polymer resin) treatments are considered acceptable in the gem trade, unless colored oils are used to change the color. For fine gemstones, treatments are required to be disclosed by the U.S. Federal Trade Commission.
Emerald is mined around the world, although certain localities provide the best gemstones. Columbia is especially notable, producing fine deep-green emeralds of excellent transparency. Brazil and Pakistan are other notable sources of fine emeralds.