A series between two end members, fayalite, an iron silicate, and forsterite, a magnesium silicate. Most minerals contain a mixture of Mg and Fe and so are called "olivine".

Fayalite due to iron: RI is higher, SG is higher, it is browner (darker) in color. a darker color than forsterite.

Olivine is found in silica poor igneous rocks. Olivine has a high melting point and is one of the first minerals to crystallize from the molten rock.

Some rocks can be composed of almost all olivine ("dunites").

Olivine is also found in meteorites. Some gem olivines were obtained from meteorites!

The gem variety of olivine is called Peridot.

Another source: Mineral Galleries.