Peridot is a transparent gem variety of olivine. Olivine is not officially a mineral but is composed of two end-member minerals: fayalite and forsterite. Fayalite is iron rich olivine, while forsterite is magnesium rich olivine. Although iron is the coloring agent for peridot, it is technically closer to forsterite than fayalite with regard to chemical composition. Olivine is an abundant mineral, but gem-quality peridot is rather rare due to its chemical instability on Earth’s surface.