Sapphire is the single-crystal form of aluminium oxide (Al2O3), a mineral known as corundum. It can be found naturally as gemstones or manufactured in large crystal boules for a variety of applications.

The corundum group consists of pure aluminium oxide. Trace amounts of other elements such as iron and chromium give sapphires their blue, red, yellow, pink, purple, orange or greenish colour. Sapphire includes any gemstone quality varieties of the mineral corundum including the red variety, which is also known as ruby.

Blue sapphires come in a wide range of hues. Titanium and iron inclusions within the aluminium oxide result in various shades of blue. Some stones are not well saturated and show tones of grey. It is common to bake natural sapphires to improve colour. This is usually done by heating the sapphires to temperatures of up to 1800°C for several hours, or by heating in a nitrogen deficient atmosphere oven for 7 days or more. On magnification the silk due to included rutile needles are visible. If the needles are unbroken, then the stone was not heated. If the silk is not visible then the stone was heated adequately. If the silk is partially broken then a process known as low tube heat was used. Low tube heat is the process where the rough stone is heated to 1300 °C for 20 to 30 minutes over charcoal. This takes out any grey or brown in the stone and improves colour saturation.