The red variety of the mineral corundum, in its finest quality the most valuable of gemstones. Only medium to dark tones of red to slightly violet-red or very slightly orange-red are called ruby; light reds, purples, and other colors are properly called sapphires. In its pure form the mineral corundum, with composition Al₂O₃, is colorless. The rich red of fine-quality ruby is the result of the presence of a minute amount of chromic oxide, usually well under 1%. The chromium presence permits rubies to be used for lasers producing red light.