An opal is a cryptocrystalline variety of porous hydrated silica, deposited from aqueous solution in all kinds of rocks, and also formed by replacement of other minerals. Opals are variously colored; the best gem varieties are translucent, with milky or pearly opalescence and iridescence due to light scattering and interference from internal cracks and cavities. Common opal is used as an abrasive, filler, and insulator. Hardness 5.5–6.5; relative density 2.0.