Olivine is found mainly in ultramafic igneous rocks. It also occurs in marble formed from metamorphosed impure limestone. Associated minerals include augite, chromite, diopside, feldspar, hornblende, plagioclase and spinel. Olivine is a mineral group rather than a species. The term “olivine” is used for a solid solution series of silicate minerals that contain varying amounts of iron and magnesium. The two end members are fayalite, the iron-rich species, and forsterite, the magnesium variety.

These formulae are mostly theoretical as nearly all fayalite contains some magnesium, and forsterite is practically never iron-free. As a result, it is almost impossible to determine the point in the series at which one of these minerals ends and the other begins, hence, the blanket term olivine.

Olivine minerals are hard and highly resistant to heat. They are therefore used as abrasives and refractory sands and some are mined for their magnesium. Olivines are among the first minerals to crystallize from mafic magma. Good crystals are tabular or box-shaped, but those are rare. The minerals are often found as grains in alluvial gravels and basaltic lavas.

The most valuable gem-quality olivine is known as peridot. Most peridot is magnesium-rich forsterite, its color caused by the presence of up to 15 percent iron, together with nickel and chromium as trace elements.

Timothy Mountain east of Lac La Hache produces bombs (blobs of lava ejected violently from a volcano, often cooling in flight), from which excellent dark green stones weighing as much as 10 carats have been obtained. Lightning Peak in the Monashee Mountains produces peridot grains large enough to cut into small gems.

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