Gamet peridotite: Most peridotite has traces of garnet or spine! but sometimes, especially in Iherzolite, small crystals can form like cherries in a bun. Some are big enough to chip out as gems Garnet and spinel peridotites, or rather Iherzolites, are found as xenoliths in basaltic rocks and kimberlites, and are thought to form in the upper mantle.