Wolframite. Credit: Mineral Information Institute.
Wolframite is iron manganese tungstate (Fe, Mn)WO4, the chief ore of tungsten. Wolframite forms brown to black monoclinic crystals or granular masses. It is found in quartz veins and pegmatites associated with granitic rocks and also in high-temperature hydrothermal veins in association with other minerals, including tin ores. China is the main producer. Hardness 5–5.5, relative density 7–7.5.