Ruby

Ruby is corundum, an aluminium oxide mineral, in its red form: a trace of chromium supplies the colour, and the name goes back to the Latin word for red. Gem corundum in any shade of red, pink included, is called ruby, while the remaining colours are sold as sapphire. It is very hard, at 9 on the Mohs scale, and some stones carry a star of light when cut as a domed cabochon. Nearly every ruby now sold has been treated, usually by heating, and synthetic ruby, made since the nineteenth century, was the heart of the first working laser in 1960. The Mogok valley in Burma was for centuries the main source. Red spinel was long confused with it, and some famous 'rubies', the Black Prince's Ruby among them, are really spinel. Wikipedia records a Hindu astrological belief tying ruby to the Sun and the god Surya. Cunningham pairs ruby with fire and Mars, treats it as a stone for wealth and protection, and says it was worn to lift sadness, bring joy and confidence, and keep nightmares away from sleep; he notes it was thought a fitting offering to Krishna and the Buddha. Hall calls it a stone of energy and abundance and a shield against psychic attack, though she adds that it may overstimulate delicate or irritable people.