Class : Silicates
    Subclass : Nesosilicates
    Crystal system : Orthorhombic
    Chemistry : X₂SiO₄
    Rarity : Very common

The term olivine refers to a group of minerals of general formula X₂SiO₄, where X is Ca, Fe, Mg or Mn, which gives rise to several series or continuous solid solutions ; the most widespread being the ferromagnesian forsterite (Mg) - fayalite (Fe) series. Gemmy quality forsterite is also called peridot. Olivine owes its name to its olive-green color. Olivine is constitutive of most ultra-basic rocks (peridotites) and common accessory components of basic magmatic rocks (basalts and gabbros). Subject to the considerable pressures prevailing in the Earth's mantle, the structure of olivine changes and adopts a denser spinel-like structure. This stability of olivine added to its presence in very many meteorites suggests that it is the dominant mineral of the mantle and therefore probably the most abundant terrestrial mineral ! More rarely olivine is a mineral resulting from the thermal metamorphism of some dolomitic limestones, in this case it is always magnesian. The olivine group, however, is not restricted to the only solid Fe-Mg solution. Other cations can integrate the structure :

In France, forsterite is common in centimetric crystals in many basalts of the Massif Central (Suc d'Eyme, Grand Champagnac, etc...). We should also note the very beautiful automorphic olivine crystals of Mont Croustet, they constitute pretty olive green to brown tabular floaters with perfect shapes (photo on the right) and which can reach dimensions of more than 1.5 cm. Peridotite nodules of a very beautiful green are known in abundance in the basalts of Sauterre (Puy-de-Dôme) and in the projections of Mount Briançon, near Langeac (Haute-Loire).

Rare plane {011} with axes of crystals at 60°47 'or {012} with crystal axes at 30°, interpenetration or multiple twinning ; also mentioned on {100}.

Peridot is one of the rare gemstone never treated because to date no known improvement is possible. The lilypads inclusions are characteristic of peridots of volcanic origin and are natural (expansion and explosions of CO₂ inclusions during the rise towards the surface of the peridot in the magma, we speak of decrespitation) this is not linked to a heat treatment. Olivine can be imitated by glass, so be careful with large gemstones. It is a mineral with a strong birefringence, on the gems an important doubling is easily observable with a magnifying glass (doubling facets when the stone is observed by transparency as in the photo on the right).

It should also be noted that synthetic forsterites are produced, they are manufactured in the laboratory by Russians since 1999. However synthetic crystals that are cut for jewelry are blue-violet, color that does not exist for this species in nature... this synthesis is actuallycreate to imitate tanzanite...