Peridot is the official birthstone for the month of August as adopted by the American National Association of Jewelers in 1912. It is also the stone for the Zodiac sign of Libra. Peridot may be given as a gem on the 16th wedding anniversary. 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.