This variety has only been known and appreciated since the beginning of the twentieth century, after the discovery of the Australian deposits. Before that. it was only found occasionally, as a true rarity, and was regarded with some mistrust. In some cases it was known to be obtained by artificial coloration of the mass, using the same procedures used for agate. to highlight the patches of iridescence.
Appearance It has a bluish gray. smoke gray to black color. This dark ground greatly enhances its appearance by emphasizing the patches of color caused by diffraction. The range of colors and shapes of the patches are roughly the same as for white opal, and the standards and terminology employed are identical. Here, too, when the patches are angular, polygonal, clear-cut, and of uniform size. it is called (black) harlequin opal, and this is the most valuable type of all.
Naturally, the cabochon cut is used and thicker gems are preferred, those of regular outline, better suited to jewelry, being the most highly prized.
Distinctive features Like white opal, it is unmistakable. Only the dividing line between black and white opal is not clear—cut, one type merging into the other. Care needs to be exercised to ensure that what may appear to be a black opal is not really a thin layer of white opal on a piece of dark-colored rock. Examination of the stone from the side will reveal this combination.
Occurrence Black or dark opal comes alrnost exclusively from Australia, the main deposit being at Lightning Fiidge (New South Wales); less important sources are Tintenbar (New South Wales) and Mintable (South Australia). Very small quantities also come from lndonesia.
Value Very high; the highest for opal, and therefore immediately after the principal gemstones (diamond, emerald, ruby, and sapphire). Naturally, the value is slightly lower when the contrast between the iridescent patches and background color is less pronounced, when the patches are less clearly defined, and when the range of colors is limited; or. finally, when the patches are arranged haphazardly, are uneven, or at any rate less attractive.Nevertheless, all black opal is expensive.
Simulante and Synthetice Black opal is harder to imitate than white opal. The French company which produces synthetic While 0Dal has succeeded in producing very attractive black opal, but at a very high cost, which few are willing to pay for a synthetic stone because, for the same price, one could obtain quite a fine, natural white opal. Because of the value of black opal, even very thin veins are used for ornamental purposes. By reason of opal’s extreme brittleness, it is used either as the base of doublets with the top. convex part of colorless quartz, or as an intermediate layer of triplets, the base of which consists of common opal and the top part of quartz. These doublets and triplets are not regarded as false, but are obviously much less valuable than all-opal cabochons. But when. as sometimes happens, the triplet is made with white opal, the underside of which is cemented to the bottom layer with black glue, to make the entire gem look more valuable, this is a clear case of falsification. Recently, treatment of white opals has recommenced, taking advantage of their natural porosity to darken their color, and thereby increase their value. These processes are very hard to detect.