One of the main ores of tungsten, like many metal minerals it forms a solid solution between a manganese (in this case, though magnesium is more common) and an iron end member (called wolframite), where the individual crystal can occupy any place along the spectrum of compositions between them. The crystals have a very bright lustre, and range from opaque black, through dark yellowish brown to a deep and dark transparent red like the specimen in the photo. Crystals are often flat tablets with lines down the long axis called striations. Like many metallic ores it is very dense in the hand, and fairly soft, with a hardness on Mohs scratch scale of 4.5.
It forms in high temperature hydrothermal veins, where the transporting superheated water was abruptly cooled, and in altered granites or pegmatites. The type locality is in Nevada, and it was named after a German mining engineer. It is nearly always found in association with quartz in veins, as in the Peruvian specimen (5.1 x 4.5 x 3.1 cm) in the photo, though there is also some associated calcite. As well as this locale, the Sweet Home mine in Colorado (also famous for its rhodochrosite). Bolivia and the Czech Republic are all notable sources.
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Image credit: Rob Lavinsky/iRocks.com
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