Common Opal varies from transparent to opaque, being most often translucent and sometimes exhibiting the peculiar milkiness of color which we call opalescence. It has sometimes a glassy, but often a waxy luster, the latter when pronounced giving rise to the varieties known as wax Opal and resin Opal. When Opal has the banded structure of agate it is known as Opal-agate; when it has the color of jasper as jasper Opal, and when that of chrysoprase as prase Opal. But none of these varieties are used in any quantity as gems. This distinction is reserved almost wholly for the variety known as noble or precious Opal. This is Opal which exhibits a play of colors. No essential chemical or physical distinction between noble Opal and other varieties is known. In a large vein of Opal portions will exhibit the play of colors and the remainder will not, but why the difference has not yet been determined. Neither can the origin of the varied coloring; i. e., the iridescence, be determined. Some regard it as due to interspersed layers containing different percentages of water, which break up the rays of light somewhat as a prism does, while others think that minute cracks and fissures through the stone furnish surfaces from which the rays are reflected in different colors back to the eye. Some Opals which are dull and lusterless when dry exhibit considerable play of color when immersed in water, and this fact seems to favor the first theory of the cause of the iridescence, but the subject is not understood. The character of the play of colors differs in different Opals, and this gives rise to different varieties. The true noble Opal has the color quite uniformly distributed. When the color appears in flashes of red, yellow, etc., the stone is known as fire Opal; if blue as girasol, and if chiefly yellow as golden Opal. When the patches of color are small, angular and uniformly distributed it is called harlequin Opal, and if these are long and somewhat parallel, flame Opal. These colors are not, of course, inherent in the stone, its color varying from colorless to opaque white. The black Opals sometimes seen and highly prized by some are usually artificial, and are made by soaking ordinary Opals in oil and then burning oil on them. The brilliancy of the stone is thus increased, but it is usually fragile and liable to lose color.