Cassiterite is a tin oxide mineral, and is the primary ore for tin. In addition to its important industrial use, it has some interesting properties that make high quality crystals valuable as gemstones. Cassiterite specimens are also very popular with mineral collectors.
Cassiterite is reasonably hard, with a rating on the Mohs scale of 6 to 7. It tends to be opaque though thin crystals can be translucent. Its chief virtue is its multiple crystal faces and outstanding luster. Many specimens are graded as having "adamantine" (diamond-like) or submetallic luster.
With a specific gravity of 6.7 to 7.1, cassiterite is one of the densest gem materials known. By way of comparison, the very dense hematite has a specific gravity of around 5.28, while sapphire is about 4.03 and diamond is 3.53. Cassiterite also has a very high refractive index of 1.997 to 2.098, higher than zircon, sphene and demantoid garnet (though not quite as high as diamond).
The colors of cassiterite range from purple, wine, black, reddish brown to yellowish brown. The dark brown and blackish hues are the most typical.
Cassiterite is found in hydrothermal veins and pegmatites associated with granite intrusions. Because of its durability it is also frequently found concentrated in alluvial placer deposits, sometimes in large enough quantities to be commercially exploitable (as in Malaysia, for example).
Deposits of cassiterite are found in Australia, Bolivia, China, Congo, Czech Republic, England (Cornwall), Malaysia, Mexico, Namibia, Peru, Spain and the USA (California). The finest cassiterite crystals have traditionally come from the Bolivian deposit with its hydrothermal veins. Some recent facet-grade material from China rivals the Bolivian cassiterite, and displays unusual golden hues.
Cassiterite from Australia