Onyx is an agate gemstone formed from alternating bands of the silicate mineral chalcedony. A cryptocrystalline compound with intergrown quartz and moganite crystals, the chalcedony forms as a precipitate at low temperatures as existing rock breaks down. Throughout history, onyx has been used in jewelry, décor, and hardstone carvings including cameos (layered carvings with relief images that contrast in color against their background). Onyx can be found in every color of the rainbow, although black and red onyxes are rarer.