The sapphire, birthstone for September, is a relative of July's birthstone, ruby.
Red corundum is called the ruby, while all other gem quality forms of corundum are called sapphires.
Typically, sapphires appear as blue stones, ranging from very pale blue to deep indigo, due to the presence of small amounts of titanium and iron within the crystal structure. The most valued shade of blue is the medium-deep cornflower blue. These different colors are caused by different kinds of impurities within the crystal. For example, yellow sapphires get their color from ferric iron, and colorless gems have no contaminants. It was the gem of Apollo, Greek God of prophesy and was worn by his worshipers visiting his shrine in Delphi to seek his help.