Ruby

Ruby is the red gem variety of corundum, aluminium oxide. Its red comes from traces of chromium, the same element that fluoresces under ultraviolet light and gives the finest rubies their glowing 'pigeon's blood' colour. Any corundum that is not red is classed instead as sapphire.

Properties

Corundum is trigonal and, at 9 on the Mohs scale, is the hardest natural material after diamond, which makes ruby exceptionally durable. Crystals are typically tabular or barrel-shaped; rutile inclusions ('silk') can produce a six-rayed star in cabochon-cut stones.

Occurrence

The historic source is the Mogok Stone Tract of Myanmar; Mong Hsu (Myanmar), Mozambique, Thailand, Sri Lanka and Madagascar are major modern producers. Most rubies on the market are heat-treated to improve colour and clarity.