The chemical formula for sapphire is Al2O3. It is aluminum oxide and the mineral corundum, a common substance in the earth's crust. Opaque varieties of corundum form the abrasive in sandpaper, but clear varieties make up the gemstones ruby and sapphire.
According to Reference.com, the transparent gem varieties of corundum are colorless, pink, red, blue, green, yellow, and violet. While the traditional color of sapphire is blue, with the exception of red, jewelers sell all these varieties as sapphire.
Trace amounts of metals mix with corundum to give it its color. Star sapphires contain particles of the mineral rutile. Jewelers cut and polish these sapphires so the rutile reflects light to produce a star-shaped pattern.
Corundum is second only to diamond in hardness and by definition is the standard for a 9 on the Mohs hardness scale. It scratches practically all other minerals, leaving a white streak.
Manufacturers create synthetic corundum using flame fusion. The process involves heating barium fluoride and aluminum oxide, along with the appropriate trace metal to produce the desired color. At 2000 Celsius, this synthetic process allows jewelers to produce flawless single-crystal corundum gems which are much larger than those normally available in nature. Manufacturers use the same process to produce most industrial corundum.