Ruby is one of the highest valued coloured gemstones, in fact, large rubies can fetch higher prices than equivalently sized diamonds. Ruby has been prized for centuries because of its excellent Mohs scale hardness of 9, along with its treasured rich red hue and vitreous lustre. Ruby is a variety of corundum that gets its red colour from chromium. Corundum that occurs in other colours is classified as sapphire. In its pure form, corundum is colourless. The word "corundum" comes from the Tamil "kurundam", meaning "ruby sapphire".