CASSITERITE
specimen from Walls Lode, Old Vitifer Mine, Postbridge, Dartmoor, Devon
SnO2 (Tin Ore)
This mineral may form as short or slender prismatic, or bipyramidal srystals. Other habits are massive, granular, botyoidal and reniform. Typically, it is brown to black, but it may also be yellowish or colourless. The streak is white, grey or brownish. Cassiterite is transparent to nearly opaque. The lustre is adamantine on crystal faces, and greasy when fractured. It forms in high temperature hydrothermal veins, where associated minerals include quartz, chalcopyrite and tourmaline. It als forms in some contact metamorphic rocks.
Hardness: 6-7