Turquoise, the robin's egg blue gemstone worn by Pharaohs and Aztec Kings, is probably one of the oldest gemstones known. Yet, only its prized blue color, a color so distinctive that its name is used to describe any color that resembles it, results in its being used as a gemstone. Turquoise has been, since about 200 B.C., extensively used by both southwestern U.S. Native Americans and by many of the Indian tribes in Mexico.