Ruby is the usual name for transparent red corundum. Ruby is red or pink. The name is derived form the Latin word ruber, which describes the reddish colour of the stone.

India was regarded as the ruby’s classical country of origin for a long time, and ruby along with other gemstones, particularly emeralds and sapphires, is found in the fine jewellery of the maharajahs and maharanis. The Sanskrit word for ruby is 'ratnara', which translates loosely as 'king of the gemstones'. Hindus believed that the colour of rubies was due to an inextinguishable flame that burned in the stone, which would even cause water to boil. Because it was thus very highly prized, any particularly beautiful ruby crystal was ‘welcomed’ with great pomp in ancient India: the ruler sent high dignitaries out to meet the precious gemstone and welcome it in appropriate style. Even nowadays, rubies still decorate the insignia of many royal households.

Historically, the best rubies have come from a small (around 400 square miles) area near Mogok, Myanmar (previously Burma), where the stones are sieved from limestone gravel. Rubies are believed to have been mined from Myanmar from prehistoric times. There is certainly considerable evidence that they were worked during the times of Marco Polo. Another important source of ruby is Thailand (previously Siam). These rubies tend to differ from those found in Myanmar in that they are usually paler, being pink rather than red. They are also found in Cambodia, Ceylon and Franklin North Carolina.

Legends and sayings have grown up about rubies all over the world: The Native Americans believed that offerings of a fine ruby would result in rebirth as a powerful emperor, while in the Middle Ages the blood-like colour of the stone led to the belief that a ruby would protect the wearer from injury. Ruby is also the most commonly named precious stone in the Bible. Proverbs 31, for example, states that: "A virtuous wife is worth more than rubies."

A synthetic ruby crystal was used to create the first laser, and the highest price ever paid for a ruby was $3.6 million, for a 15.97 carat ruby in 1988. Spinel and garnet are frequently confused with ruby – the well known Black Prince’s ruby and Timur ruby are in fact both spinels.