This is the variety which has been known and used for the longest time, and was. up to the end of the nineteenth century, the noble opal par excellence, praised by Pliny the Elder as the ultimate in gemstones, due to its marvelous and mysterious, iridescent qualities.
Appearance It has a whitish (watered—down milk) to light grayish, dull yellow, light blue-gray, or pale-blue ground color. The range of colors of the patches, due to diffraction, depends on the size of the minute spheres of which the gem is composed: patches will be violet to blue for structures with very small spheres, gradually turning to green, yellow, orange, and red as the size of the spheres increases. The wavelengths diffracted depends on the distance between rows of spheres. When this becomes too great, diffraction no longer occurs. The patches of color can be more or less clearly defined, extensive, and homogeneous in size. Pieces with angular, polygonal, evenly distributed patches in a wide range of color with clear-cut edges are known as harlequin opals: this is the most valuable variety, for both black and white opal. Varieties with heterogeneous patches of color and poorly defined edges are less highly prized.
White opal is, whenever possible, cut into fairly convex cabochons. it is sometimes cut flat but here there is a risk of breakage, as the stone is brittle. The most valuable cabochons are the strongly curved, oval ones. Many others have a vaguely triangular shape with rounded corners or, at any rate, less symmetrical shapes, which permit a higher yield from irregular rough stones.
Distinctive features White opal is unmistakable, more immediately recognizable than any other gem. There is, of course, the problem of distinguishing it from black opal. given the wide range of gradations from one to the other. It is called black when the background color is mid-gray, smoke gray. blue or black; otherwise it is called white.
Occurrence As already mentioned, white opal has been mined for centuries in Czechoslovakia; but production there is nowadays extremely limited. Most white opal now comes from Australia, mainly from the Andemooka and Goober Pedy deposits in South Australia. Light-colored opal, including some fine opal, with the iridescence of noble opal, also comes from Mexico.Smaller quantities are also extracted in the United States.Brazil, Japan, and Indonesia.
Value The best quality gems fetch very high prices, exceeded only by the four principal gemstones (diamond. emerald, ruby, and sapphire), imperial jade, alexandrite. and black opal. The potential price range, however, is very extensive and hard to quantify without direct experience. Many specimens which are very pleasing in appearance are quite modestly priced, costing no more than other secondary gems. Specimens with weak, barely visible iridescence are fairly cheap.
Sirnulants and synthetics it was long considered impossible to imitate opal. In recent years, however, an imitation, which. at first sight. looks deceptively similar to opal has appeared on the market. It is called "Slocum stone." after its inventor. A plastic imitation has also recently come onto the market. lt is very similar indeed to the natural stone, being composed of microscopic spheres, like opal.Fortunately, its low melting point, hardness, and density can distinguish it, despite its appearance. White opal has been manufactured synthetically by a French company for nearly a decade; the appearance of its iridescent patches is fairly distinctive (each being in the form of a mosaic), but it can nonetheless only be distinguished by an expert.