White precious opal

 

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 grounc' 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 tc 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 toc 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-cuedges are known as harlequin opals: this is the most valuable variety, for both black and white opal. Varieties witF 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 c• breakage, as the stone is brittle. The most valuable cabochons are the strongly curved, oval ones. Many otherhave a vaguely triangular shape with rounded corners or at any rate, less symmetrical shapes, which permit a highe, yield from irregular rough stones.


Distinctive features White opal is unmistakable, mor~immediately recognizable than any other gem. There is, c' course, the problem of distinguishing it from black opa given the wide range of gradations from one to the other i is called black when the background color is mid-gra> smoke gray, blue or black; otherwise it is called white.

Occurrence As already mentioned, white opal has bee mined for centuries in Czechoslovakia; but productio there is nowadays extremely limited. Most white opal no.. comes from Australia, mainly from the Andemooka ar Coober Pedy deposits in South Australia. Light-colore opal, including some fine opal, with the iridescence
noble opal, also comes from Mexico.Smaller quantities are also extracted in the United State Brazil, Japan, and Indonesia.

Value The best quality gems fetch very high prices, e• ceeded only by the four principal gemstones (diamor, emerald, ruby, and sapphire), imperial jade, alexandrit and black opal. The potential price range, however, is ve, extensive and hard to quantify without direct experiencMany specimens which are very pleasing in appearaf are quite modestly priced, costing no more than other s ondary gems. Specimens with weak, barely visible irid,cence are fairly cheap.


Simulants and synthetics It was long considered possible to imitate opal. In recent years, however, imition, which, at first sight, looks deceptively similar to o; has appeared on the market. It is called ' Slocum stonF after its inventor. A plastic imitation has also recently cc onto the market. It is very similar indeed to the natw stone, being composed of microscopic spheres, like or Fortunately, its low melting point, hardness, and dens can distinguish it, despite its appearance. White opal h been manufactured synthetically by a French company nearly a decade; the appearance of its iridescent patcr is fairly distinctive (each being in the form of a mosaic), r it can nonetheless only be distinguished by an expert.

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