Manganite. Image source: Mineral Information Institute, courtesy of the Smithsonian Institution.
Manganite is a black mineral consisting of hydrated manganese (III) oxide (MnO[OH]); an ore of manganese. It forms as low-temperature veins and secondary deposits, and occurs as bundles of monoclinic system striated prisms, which are often crusts of small crystals. It is black or gray in color and has a metallic luster. It alters to pyrolusite. Manganite is found in western Europe, Michigan and California. Hardness 4, relative density 4.3.