Peridot is a gem-quality olivine and a (Mg, Fe)2SiO4 type silicate mineral. The formulation approaches Mg2SiO4 because it is a magnesium-rich olivine (forsterite). The green colors of the gem depend on the structure of the iron contents. It occurs both in volcanic basalt and in pallastic meteorites in silica-deficient rocks. It is one of the two gems that have been observed to form in the rock of the upper mantle rather than in the earth�s crust. It�s gem-quality is unusual on the earth�s surface due to its environmental sensitivity when transporting deep into the mantle.