Peridot (/ˈpɛrɪdɒt/ or /ˈpɛrɪdoʊ/) is gem-quality olivine, which is a silicate mineral with the formula of (Mg, Fe)₂SiO₄. As peridot is the magnesium-rich variety of olivine (forsterite), the formula approaches Mg₂SiO₄.

Etymology[edit]

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 Middle English Dictionary’s entry on peridot includes several variations : peridod, peritot, pelidod and pilidod – other variants substitute y for the is seen here.^[1]

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 including peridot to the Abbey.^[2]

Geologically[edit]

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 peridotitexenoliths of the mantle, which lavas carry to the surface; but gem quality peridot occurs in only a fraction of these settings. Peridots can also be found in meteorites.

Peridots can be differentiated by size and composition. A peridot formed as a result of volcanic activity tends to contain higher concentrations of lithium, nickel and zinc than those found in meteorites.^[4]

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.

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