When people talk about Sapphire, they generally mean the natural Blue Sapphire gemstones. Sapphire is a form of Aluminium Oxide known as Corundum.
Pure Corundum is in fact clear. Blue Corundum (Sapphire) is made up of Corundum (Al2O3), Iron, and Titianium impurities (Fe2+ & Ti4+). Primarily, the Titanium is responsible for the Blue coloration.
Blue sapphire exists in various mixtures of its primary (blue) and secondary hues, various tonal levels (shades) and at various levels of saturation (brightness). Blue sapphires with no more than 15% violet or purple are generally said to be of fine quality. Sapphires also occur in shades of pink, orange, brown, yellow, green and also colorless sapphires.