Cassiterite. Image source: Mineralogical Information Institute.
Cassiterite is a mineral consisting of stannic oxide (SnO2) with iron impurity; the chief ore of tin. Cassiterite is usually translucent brown to black, with submetallic luster; it forms short tetragonal prismatic crystals, but is usually massive granular. It occurs in placer deposits, chiefly in the Malay Peninsula, and in pegmatities and other intrusive, igneous rocks. Hardness 6–7, specific gravity 6.8–7.1.