A Ruby is a pink to blood-red gemstone, a variety of the mineral corundum (aluminium oxide). The common red color is caused mainly by the presence of the element chromium. Its name comes from ruber, Latin for red. Other varieties of gem-quality corundum are called sapphires. The ruby is considered one of the four precious stones together with the sapphire, the emerald, and the diamond.

Prices of rubies are primarily determined by color. The brightest and best "red" called pigeon blood-red, commands a huge premium over other rubies of similar quality. After color follows clarity: similar to diamonds, a clear stone will command a premium, but a ruby without any needle-like rutile inclusions will indicate that the stone has been treated one way or another. Cut and carat (size) also determine the price.