Peridot (/ˈpɛrɨdoʊ/ or /ˈpɛrɨdɒt/) is gem-quality olivine. Olivine is a silicate mineral with formula of (Mg, Fe)2SiO4. As peridot is the magnesium-rich variety (forsterite) the formula approaches Mg2SiO4.
The origin of the name peridot is uncertain. The Oxford English Dictionary suggests an alteration of Anglo–Norman pedoretés (classical Latin pæderot-), a kind of opal, rather than the Arabic word faridat, meaning “gem”.
The earliest use in England is in the register of the St Albans Abbey, in Latin, and its translation in 1705 is possibly the first use of “peridot” in English. It records that on his death in 1245 Bishop John bequeathed various items to the Abbey: “He gave also three noble Rings, in one whereof is an Oriental Sapphire, of wonderful bigness: In another is the Stone call’d Peridot, in the middle whereof is set a Sapphire of great beauty; it is said to be good against the Cramp, and is made almost in the form of a Buckler: in the third Ring is also an Oriental Sapphire, but less than the former.
Peridot is one of the few gemstones that occur in only one color, an olive green. The intensity and tint of the green, however, depends on how much iron is contained in the crystal structure, so the color of individual peridot gems can vary from yellow—to olive—to brownish-green. The most valued color is a dark olive-green.
Olivine, of which peridot is a type, is a common mineral in mafic and ultramafic rocks, and it is often found in lavas and in peridotite xenoliths of the mantle, which lavas carry to the surface; but gem quality peridot only occurs in a fraction of these settings. Peridot can be also found in meteorites.
Olivine in general is a very abundant mineral, but gem quality peridot is rather rare. This is due to the mineral’s chemical instability on the Earth’s surface. Olivine is usually found as small grains, and tends to exist in a heavily weathered state, unsuitable for decorative use. Large crystals of forsterite, the variety most often used to cut peridot gems, are rare; as a result olivine is considered to be precious.
Peridot olivine is mined in Arkansas, Arizona on the San Carlos Reservation, Hawaii, Nevada, and New Mexico at Kilbourne Hole, in the US; and in Australia, Brazil, China, Egypt, Kenya, Mexico, Myanmar (Burma), Norway, Pakistan, Saudi Arabia, South Africa, Sri Lanka, and Tanzania.