Common manganese mineral, MnO₂; sp. gr. 4.5–5.0; hardness 5–6 for crystals, decreasing to 2 when massive; tetragonal; black to bluish-grey; black streak; metallic lustre; crystals rare, acicular and rod like, but it is usually massive, cryptocrystalline, and dendritic on joints and bedding planes; cleavage prismatic when present; a secondary mineral found in the oxidized zone of manganese deposits, in quartz veins, and as nodules on the sea bed. Synthetic MnO₂ is used for dry batteries, as a decolourizer for glass, and in the manufacture of chemicals.