Peridotite (World of Earth Science)
Peridotite is a dark-colored, coarse-grained igneous rock believed by many scientists to be the primary rock of the exterior of Earth's mantle. The rock typically forms in volcanic pipes and is forced to the surface from great depths during a volcanic eruption.
Peridotite consists of a dense iron and magnesia mineral called olivine, as well as pyroxenes and a small amount of feldspar. It is a pistachio-green color when fresh, but weathering creates iron oxides that turn it a medium brown. The iron and magnesia-rich rock is the most common host for naturally occurring diamonds. South African diamonds are obtained from a mica-rich form of peridotite called kimberlite. Periodite is also a source of valuable ores and minerals including chromite, platinum, nickel, and precious garnet. In rare instances, individual olivine crystals in periodite are large enough and pure enough to be designated a gem. The resulting gem is a semi-precious mineral called peridot.
Periodite is found worldwide, but particularly in New Zealand.
See also Gemstones; Igneous rocks; Plagioclase