Pyrolusite, which has the composition MnO2, is the main mineral used for manganese smelting. It contains 63.2% manganese. It belongs to the tetragonal crystal system, and the crystals have small granular or acicular shapes. It is often in a blocky aggregate form. Its colour and streaks are black, and it has a submetallic lustre, a specific gravity of 4.84, a low Mohs hardness and easily stains hands. It mainly forms in deep sediment or by the oxidation of primary manganese ore. It is an important raw material for smelting manganese.