Olivine (of which Peridot is the gemmy version) is a common mantle mineral, found on the surface in volcanic and exposed mantle rocks. It also sometimes occurs as beautiful green to brown crystals large enough to be faceted as gems, and its deep origins could induce one poetically to view it as our world's crystallised ichor. Known since antiquity, The traditional locale was St John's Island (aka Zabargad), off the Egyptian coast in the Mediterranean. In this place, a piece of Tethys oceanic crust was pushed up out of the mantle during Africa's ongoing collision with Europe, bringing its gems along with it. The earliest record of it being mined here dates from 70CE, and all the antique stones found in jewellery and crowns all over Europe came from Zabargad, though they were often mis-named as emerald. Known as Chrysolite in the ancient Greek world, ancient Egyptians prised it as the gem of the sun, and historians have speculated that Cleopatra's famous emeralds were olivine rather than beryl.
Hawaii was another historical source, where it was known as Pele's tears. It is now mostly mined out, though the cinder cone called Diamond Head was renowned for its crystals (see our past athttp://tinyurl.com/luex5gz). The grains at the famous Papakolea greensand beach near Hilo (covered athttp://tinyurl.com/q2f9zgz) are sand sized peridot crystals. The most valued colour is a bright olive green, with extra iron content increasing the brown component. The origin of the name is uncertain, but it is one of the birthstones for August, accounting for its continuing popularity.
The main current source remains Arizona, where it occurs as phenocrysts (large crystals that grew slowly before eruption) in a layer of basaltic lava at the top of Peridot Mesa on the San Carlos Apache Reservation. Only inhabitants are allowed to mine it, either in weathered out form gathered in gullies or by drilling and blasting. Other significant sources are Pakistan, China and Burma where it occurs as phenocrysts in basalts or as mantle xenoliths (foreign rocks) carried up by erupting lavas.
There is another amazing source: 4.5 billion year old crystals of peridot from pallassitic meteorites have been occasionally faceted and marketed with this extra cachet. It used to be thought they came from an asteroid's core-mantle boundary, but recent research suggests that they were formed during shattering collisions in the asteroid belt (see our post at http://tinyurl.com/mnhvap8)
Not all the gem olivine out there has its origins in the mantle however, some form in high temperature contact metamorphic deposits, when hot olivine rich magmas interact with the country rocks that they intrude. The deep green gemmy specimen in the photo bears a testimony to this origin, in the form of dark needles of the rare magnesium-iron borate mineral Ludwigite. The latter often forms when such magma meets carbonate rocks, producing a diverse metamorphic mineral assemblage known under the umbrella term skarn. Mined in Pakistan, the specimen in the photo measures 5.3 x 2.1 x 1.4 cm. Ludwigite forms in high temperature magnesium and iron rich skarns, and was first described in 1874 from the type locality in Romania and named after the Austrian chemist at Vienna university who first analysed it. It is naturally ferromagnetic and can form permanent magnets. In this case it consists of needle shaped (known as acicular) black crystals within the gemmy green peridot.
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