Sapphire, whose name comes from the Greek for 'blue stone', is a gemstone variety of a very hard mineral called corundum. It is second in hardness only to diamond. It is similar to ruby, but with other trace minerals. Sapphire can be made artificially, and it is used to make watch bearings and in electronics.

The colour of sapphire comes from tiny amounts of the impurities iron and titanium. They soak up (absorb) the yellow in white light, so that the colour that passes through is blue. (Blue is the complementary colour to yellow in white light).