Olivine occurs in a range of compositions between pure magnesium silicate (forsterite) and pure iron silicate (fayalite), and olivine composition is often given as a number from zero to 100, where 100 is pure fayalite. Forsterite is white and fayalite is dark brown, but olivine is usually green, like these specimens found in the black basalt pebble beach of Lanzarote in the Canary Islands. Olivine has a minor use as an abrasive in sandblasting. As a gemstone, olivine is called peridot.
Olivine prefers to live deep in the upper mantle, where it makes up about 60 percent of the rock. It does not occur in the same rock with quartz (except in the rare fayalite granite). It is unhappy at the Earth's surface and breaks down fairly rapidly (geologically speaking) under surface weathering. This olivine grain was swept to the surface in a volcanic eruption. In olivine-bearing rocks of the deep oceanic crust, olivine readily takes up water and metamorphoses into serpentine.