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".
In the ancient Indian language of Sanskrit, ruby is called "ratnaraj", which translates as "king of precious stones". Ancient Sanskrit texts, the Bible and other historical writings refer to ruby as a precious gem, indicating the rich history and abiding appreciation of ruby gemstones. Ancient Hindus believed that by making an offering of a ruby to Krishna, rebirth as an emperor was assured. Burmese warriors believed that rubies would make them invincible, and even inserted rubies under their skin for this purpose.
According to the story of Marco Polo, Kublai Khan offered the King of Ceylon a city in exchange for a large ruby. Medieval Europeans believed that rubies assured good health, prosperity, wisdom and a successful love life. The English name "ruby" comes from the Latin word "ruber", meaning red. The most desirable ruby colour is a rich deep red with a hint of blue that is known as "pigeon's blood". In Thailand, ruby is known as "tabtim", which means "pomegranate" in Thai. This is because these shining red gems look like the edible seed coats found inside a ripe pomegranate.