Peridot (pronounced pair-uh-doe) is the gem variety of olivine. Olivine, which is not itself an official mineral, is composed of two minerals: fayalite and forsterite. Fayalite is the iron rich member with a pure formula of Fe2SiO4. Forsterite is the magnesium rich member with a pure formula of Mg2SiO4. Olivine's formula is written as (Mg, Fe)2SiO4 to show the substitution of the magnesium and iron. Peridot is usually closer to forsterite than fayalite in composition although iron is the coloring agent for peridot. The best colored peridot has an iron percentage of less than 15% and includes nickel and chromium as trace elements that may also contribute to the color.
Most gemstones of mineral origin are formed in the earth's crust. But there are two exceptions: both peridot and diamond are formed much deeper in the earth, in the region referred to as the mantle. Peridot crystals form in magma from the upper mantle (20 to 55 miles deep), and are brought to the surface by tectonic or volcanic activity where they are found in extrusive igneous rocks. Diamonds were formed much deeper in the mantle (around 100 - 150 miles below the surface), at extreme temperatures and pressures.
Peridot is one of the few gemstones that comes in only one color. The depth of green depends on how much iron is contained in the crystal structure, and varies from yellow-green to olive to brownish green. Peridot is sometimes referred to as "the poor man's emerald." Olivine is a very abundant mineral, but gem-quality peridot is in fact rather rare.
Important peridot sources are Burma, Pakistan, China, Vietnam and the United States. The finest quality peridot has traditionally come from Mogok in Burma, though the Pakistani peridot is now highly regarded as well. The USA was for many years the largest producer of peridot. Deposits in Arizona are the major source of American peridot. Peridot Mesa, located on the San Carlos Apache Indian Reservation east of Globe in Gila County, is the most productive locality for peridot in the world. A second Arizona location from which peridot is recovered is Buell Park in Apache County, Arizona, about 16 kilometers north of Fort Defiance. However, it is estimated that 80% to 95% of the world's production of peridot comes from the San Carlos Reservation, mainly for commercial quality peridot (peridot is the birthstone for the month of August).
The largest cut peridot is 310 carats and housed at the Smithsonian in Washington, D.C.; a 192.75 carat stone belonging to czars is in the Diamond Treasury, Moscow; and a 146 carat peridot may be found in the Geological Museum in London, England. Peridot is a relatively inexpensive gemstone in small sizes, but the value goes up with stones over 5 carats, with 10-15 carat stones very rare and expensive. There is no known treatment to improve the color or clarity of peridot, so peridot is always an untreated gem.