Cassiterite is tin oxide (SnO2), a reddish brown, opaque, sometimes translucent mineral, which is the chief tin ore mined over the world. It occurs mainly in igneous rocks and its crystal symmetry is tetragonal. In this crystal group the reddish brown crystals have a pyramidal to prismatic habitat and show twinning. This specimen from Ehrenfriedersdorf in Saxony, Germany. Actual width is 120 mm.