Olivine

Olivine, a mineral consisting of a variable mixture of magnesium and iron silicates, Mg?Si04 and Fe.2Si04, crystallising in the Prismatic system, but often occurring in irregular grains, transparent when unaltered, varying in colour from olive-green to yellow, with glassy lustre, conchoidal fracture, specific gravity between 3-2 and 35, and hardness between 6'5 and 7. Olivine is an essential constituent of basalt, occurs in some gabbros, and is the main constituent of the peridotites. A pale yellowish-green transparent variety, found in the Levant, is known as chrysolite and a leek-green one as peridot. These are cut as gems. Olivine is often flawed, and is very liable to alteration by percolating waters or weathering, a change into opaque serpentine, or into limonite, spreading along the flaws, until the whole crystal may be changed.