Manganese dioxide, known by the chemical formula MnO2, is naturally gray or black in color. It is typically found as a crystalline solid in the mineral pyrolusite, which has a matte black to dark grey metallic appearance.
The compound is naturally occurring, formed by the oxidation of manganese in the environment; two oxygen atoms bond with one atom of manganese. It is used in a variety of industrial applications; it was the original compound used in the first dry cell batteries. Additionally, it is useful as a laboratory catalyst, as an secondary alloy addition to steel and can make violet colored glass. True amethyst crystals are given the purple coloration with the presence of manganese dioxide.