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 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.
Cassiterite is one of the densest gem materials known. It also has a very high refractive index, higher than zircon, sphene and demantoid garnet.
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