Topaz, TOH paz, is a white or light-colored mineral sometimes cut into gems. It is a compound of aluminum, fluorine, silicon, oxygen, and hydrogen. Its chemical formula is Al2(SiO4)(OH,F)2. Trace impurities give topaz crystals a wide variety of colors. Topaz is hard. It defines an 8 on the Mohs hardness scale, a ranking of mineral hardness commonly used by geologists. Small amounts of topaz occur in some igneous rocks, rocks that form when molten rock cools and solidifies, and metamorphic rocks, rocks that have been changed by heat and pressure.