Precious Opal:Precious opal is a type of opal that exhibits a play-of-color, meaning it displays a rainbow-like iridescence when viewed from different angles. It is considered to be the most valuable type of opal, and is often used in jewelry. Precious opal typically has a base color of white, black, or gray, and the play-of-color is usually red, green, or blue. The colors are created by tiny spheres of silica that are stacked in an orderly fashion and diffract light to create the colors that are seen.