A species or family of gemstones that includes ruby or sapphire, depending on the colour of the gemstone (which in turn depends on the trace elements present) Corundum is a very hard mineral (hardness 9); In its rare pure form, corundum is colourless and called white sapphire. Rubies contain chromic oxide, blue sapphires contain titanium along with ferrous oxide, and yellow sapphires contain ferric oxide.