Zinc sulfide is an inorganic compound with the formula ZnS, which is the main form of zinc in nature, where it mainly occurs as the mineral sphalerite. Although the mineral is black owing to impurities, the pure material is white and is in fact used widely as a pigment. ZnS exists in two main forms, and this dualism is often a textbook example of polymorphism. In both polymorphs, the coordination geometry at Zn and S is tetrahedral. The cubic form is more stable and is known also as zinc blende or sphalerite. The hexagonal form is known as the mineral wurtzite, although it also can be produced synthetically. It is used combined with solid lubricants in friction materials.