From One Mineral, Brilliant Reds and Bright Blues

A. Both rubies and sapphires are mostly the same clear mineral: aluminum oxide, in a crystalline form called corundum. Trace elements present in varying amounts cause the different colors by altering which wavelengths of light are absorbed and reflected by the stone.

For rubies, the ideal composition includes significant traces of chromium but little or no iron. The chromium alters the crystal structure so that all but red light tends to be absorbed.

Chromium also absorbs ultraviolet wavelengths of light, but re-emits some of it at a lower energy in the red wavelengths. Iron would absorb this light; without it, the finest rubies are fluorescent in the red range.

For blue sapphires, traditionally the most sought-after color, the important trace elements are iron and titanium. Their presence alters the crystal structure so that all but the blue wavelengths are absorbed.

Sapphires in other colors have other chemical signatures. Laboratories often can identify the source of a gemstone by analyzing its chemistry.

Recent research suggests that traces of silicon, too, play a role in determining the color of corundum, but its exact role has not been determined.