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Most gemstones form in the Earth’s crust. Only two gemstones form in the Earth’s mantle – the diamond and the peridot. These are gems that crystallize at extremely high temperatures.
Gem miners find peridot as irregular nodules (rounded rocks with peridot crystals inside) in some lava flows in the United States, China, and Vietnam and, very rarely, as large crystals lining veins or pockets in certain types of solidified molten rock. Geologists believe both types of deposits relate to the spreading of the sea floor that occurs when the earth’s crust splits, and rocks from its mantle are pushed up to the surface.
Rarely, peridot can have an extraterrestrial source, being contained in meteorites that have fallen to earth. The stony-iron meteorites known as pallasites are famed for the areas of yellowish green olivines they contain.
Peridot rates 6.5 – 7.0 on the Moh’s hardness scale which is fine for daily wear. Peridot is rarely treated, but might have fractures that can be filled to improve the apparent clarity. Clean peridot only with warm soapy water. Ultrasonic and steam cleaners are never recommended.