A mineral having composition MnO₂. Pyrolusite is the most important ore of manganese. It crystallizes in the tetragonal system but well-developed crystals (polianite) are rare. It is usually in radiating fibers or reniform coatings. The hardness is 1–2 on Mohs scale (often soiling the fingers) and the specific gravity is 4.75. Crystals of polianite show a perfect prismatic cleavage and have a hardness of 6 and a specific gravity of 5.1. The luster is metallic and the color iron-black. It frequently forms pseudomorphs after other manganese minerals, notably manganite.