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Alumina
Sapphires are made from a mineral called corundum, the most common form of alumina (aluminum oxide). The chemical formula is Al2O3, meaning that for every two atoms of aluminum, there are three atoms of oxygen. These atoms are all arranged in a tightly-packed hexagonal structure, with each aluminum atom connected to eight neighboring oxygen atoms. This arrangement is also called alpha-alumina and is extremely hard -- diamond is the only mineral harder than corundum.
Impurities
Sapphires, rubies and topaz are all made from corundum. The differences in color and other properties, however, stem from the presence of trace impurities. A ruby contains chromium in place of some of the aluminum atoms. Topaz has some iron ions incorporated into its structure. Blue sapphires feature titanium and iron (II) (iron ions with a +2 charge, as opposed to the +3 iron ions in topaz). Yellow sapphires, by contrast, feature iron (II) ions -- only without the titanium found in blue sapphires.
Features
Generally, the trace impurities like the iron in yellow sapphires is less than three percent of the mass of the total crystal -- a very small amount, even though it accounts for the color that makes these stones so valuable. Heating or irradiating yellow sapphires and other colors can alter their colors somewhat, as iron atoms become oxidized (lose electrons) to gain a more positive charge. Regardless of color, sapphires typically exhibit a six-sided barrel shape, in contrast to rubies, which exhibit a hexagonal prism shape.
Synthetic Sapphires
Synthetic sapphires can be made in the lab through one of two processes: flame fusion and flux growth. Large quantities of sapphires are currently produced for use by industry and as gems, and the color can be determined by deciding which impurities to include. This process was actually invented back near the beginning of the twentieth century, and at the time it caused significant losses for gem mines in the British colonies.