Cassiterite is a mineral with a hardness of 7 out of 10 on the Mohs scale of mineral hardness [?]. These Tetagonally structured gems are made of tin dioxide, their full chemical compound being SnO2.
Cassiterite may form as short or slender prismatic, or bipyramidal, elbow-shaped crystals. It has a crystal surface reflecting light as in a mirror. They may occur as massive, granular, botryoidal, and reniform. When found in granular, banded, fibrous masses, it looks like wood. When found in groups of large bright crystals, it is usually called "diamond tin." A red variety cut for collectors is known as "ruby tin."
Typically it is brown to black, but also may be yellowish or colorless. The streak is white, grey, or brownish. It is transparent to nearly opaque. The lustre is adamantine on crystal faces, and greasy when fractured. Colorless specimens rarely occur, with most specimens mostly in brown shades.
It forms in high-temperature hydrothermal veins, where associated minerals include quartz, chalcopyrite, and tourmaline. The largest deposits are sedimentary, and occur in river or marine placers.
It is insoluble in acids and is also infusible.
It has an unusually high specific gravity for a non-metallic mineral, and has a high resistance to weathering, thus often found in stream and beach deposits.
It can be distinguished from brown diamond, brown zircon and sphene by its greater density; from zircon by the absence of any absorption lines.
Fine crystals are found in the Erzegebirge (Germany) and Cornwall (England). Orange to purple cassiterite has been found in Sri Lanka and a botryoidal laminated variety in the Bolivian tin deposits.
The name cassiterite is derived from the Greek word for "tin." The deposits in Cornwall, England, were the main source of cassiterite until the nineteenth century, a source that has been known since Roman times.
Cassiterite is an important and only abundant ore of tin, and is usually called tinstone, tin spar, tin ore, tin oxide, or black tin. It is used in the metal-plating industry and in various alloys.
As used for industry, it comes from ore deposits in Malaysia, Sumatra, the Soviet Republics, China, Bolivia. In the USA it is found in permatite veins in Oxford County, Maine, and South Dakota, but is one of the few minerals that does not occur in great quantities in the United States. It is also found on the island of Elba, Livorno, and in permatite at Piona (Italy).
Some gem-quality cassiterite occurs especially in Spain and Namibia, but most is collected for the elbow/knee-shaped crystal twins (geniculate twins) that are common. A reniform reddish variety, known as "toad's-eye tin" is usually cut cabochon.
It is rarely fashioned, but prized by collectors, because of its high brilliancy and dispersion.