The rich red color arises from the substitution of a small number of aluminum atoms by chromium atoms. Pure corundum is a colorless, trigonal crystal, but it occurs in a wide variety of colors due to infiltrations of other elements. All color variations of corundum, with the exception of ruby, are called sapphires. Such stones are called star rubies. When exposed to high temperatures, rubies turn green, but they regain their original color after cooling.
Some rubies phosphoresce with a vivid red glow when illuminated by ultraviolet light.
The most famous, and possibly oldest, source of fine rubies is Burma, now called Myanmar. Stone Age and Bronze Age mining tools have been found in the ancient mines of Mogok. Rubies are currently being mined in other areas of Southeast Asia, as well as Africa and other parts of Asia. Many gemstones that are not rubies are also called rubies. The balas, or balas ruby, for example, is a type of spinel.