AUGUST BIRTHSTONE—PERIDOT 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 belie