Polymorphic minerals have the same chemical composition but different crystal structures. (The word "polymorphic" means "many forms" in Greek.) These structures may have vastly different properties. Polymorphism occurs because the minerals crystallized under different temperature and pressure conditions. The most well-known example of polymorphism is carbon: it occurs both as graphite - a soft, opaque mineral that splits into layers - and diamond - transparent and the hardest substance on Earth. Silicon dioxide has several polymorphs: quartz, tridymite, cristobalite, coesite, stishovite, and seifertite. The last three of these polymorphs form only under extreme conditions of temperature and pressure - in meteorite impacts in nature or experiments in the laboratory.