Olivine (Magnesium Iron Silicate)

This stone is also called “evening emerald”, and the name “olivine” was given by A. Verner in 1790 to denote a green variety of the mineral, occurring in the basalts.

Magnesium iron silicate. The series ranges from the magnesium end member, Forsterite (Magnesium silicate), through the intermediary member, Olivine (also known as Chrysolite), to the iron end member, Fayalite (Iron silicate). Olivine is not scientifically classified by the IMA as an individual mineral species, but is rather recognized as a mineral group with the Forsterite and Fayalite end members. Fayalite and Forsterite create a solid solution series, and most specimens identified as Olivine fall somewhere in between this series, almost always leaning more towards Forsterite with a greater content of magnesium. Pure Forsterite is uncommon, and pure Fayalite is very rare.

Peridot is the greener type of gem-quality specimen of Forsterite-Olivine

olivine crystal on massive black magnetite-matrix

http://www.minerals.net/mineral/olivine.aspx
http://stonecontract.eu/wiki/about-gem-stone/about-olivine/