Igneous rocks are rocks formed from the cooling and solidification of the magma deep within the Earth's crust. In general, the longer and slower magma cools, the larger the resulting crystal formation will be. These long and slow crystallization processes often yield enormous crystals of gem-grade minerals, such as aquamarine, topaz, red beryl, tourmaline, citrine, amethyst, amazonite, spodumene, spessartine and feldspars. Igneous rocks also formed from some of the most valuable gemstones on Earth, including diamonds, pyropes, sapphires and olivines. These gemstones are crystallized deep within Earth's mantle or in the lower sections of the Earth's crust, and are embedded in rocks such as kimberlites, peridotites or basalts, and brought to the surface by magma eruptions.