Emerald is a gemstone and a variety of the mineral beryl (Be3Al2(SiO3)6) colored green by trace amounts of chromium or sometimes vanadium.

A fine emerald must possess not only a pure verdant green hue as described below, but also a high degree of transparency to be considered a top gemstone.

[8] This member of the beryl family ranks among the traditional "big four" gems along with diamonds, rubies and sapphires.

[9] In the 1960s, the American jewelry industry changed the definition of emerald to include the green vanadium-bearing beryl.

Thus, if an emerald has no visible inclusions to the eye (assuming normal visual acuity) it is considered flawless.

Stones that lack surface breaking fissures are extremely rare and therefore almost all emeralds are treated ("oiled", see below) to enhance the apparent clarity.

Most emeralds are oiled as part of the post-lapidary process, in order to fill in surface-reaching cracks so that clarity and stability are improved.

Other liquids, including synthetic oils and polymers with refractive indexes close to that of emeralds, such as Opticon, are also used.

[14][15] These treatments are typically applied in a vacuum chamber under mild heat, to open the pores of the stone and allow the fracture-filling agent to be absorbed more effectively.

[19] Emeralds in antiquity were mined in Ancient Egypt at locations on Mount Smaragdus since 1500 BC, and India and Austria since at least the 14th century AD.

[27] Rare "trapiche" emeralds are found in Colombia, distinguished by ray-like spokes of dark impurities.

[29] Emeralds are found all over the world in countries such as Afghanistan, Australia, Austria, Brazil,[30] Bulgaria, Cambodia, Canada, China, Egypt, Ethiopia, France, Germany, India, Kazakhstan, Madagascar, Mozambique, Namibia, Nigeria, Norway, Pakistan, Russia, Somalia, South Africa, Spain, Switzerland, Tanzania, the United States, Zambia, and Zimbabwe.

More recent studies using energy-dispersive X-ray spectroscopy methods have uncovered trace chemical element differences between emeralds, including ones mined in close proximity to one another.

[34] Hydrothermal synthetic emeralds have been attributed to IG Farben, Nacken, Tairus, and others, but the first satisfactory commercial product was that of Johann Lechleitner of Innsbruck, Austria, which appeared on the market in the 1960s.

Later, from 1965 to 1970, the Linde Division of Union Carbide produced completely synthetic emeralds by hydrothermal synthesis.

According to their patents (attributable to E.M. Flanigen),[35] acidic conditions are essential to prevent the chromium (which is used as the colorant) from precipitating.