Cassiterite
THOUGH usually opaque, this oxide of tin, corresponding to the formula SnO2, has occasionally, but very rarely, been found in small, transparent, yellow and reddish stones suitable for cutting. The lustre is adamantine. The refraction is uniaxial in character and positive in sign, the ordinary index being 1·997 and extraordinary 2·093. The specific gravity is high, ranging from 6·8 to 7·1. The hardness is on the whole less than that of quartz, being about 6 to 7 on Mohs’s scale.