Black opal

Black opal is an exceptionally beautiful gemstone, the most valuable variety of opal. Has an amazing, almost pure spectral color game. Two of the same ones do not exist: each has its own unique beauty.

Black opals are only called such, in fact they are often dominated by dark blue and green areas of the painting. Highly sought after gems with Golden and red color zones. The main advantage of black opal is a multicolored flash, which, like the lightnings sparkle on the surface. The brighter and richer this game of colors, the more expensive the stone. Best Australian gems are valued in the thousands of dollars per carat.

Color opals are usually "pure", that is, the wavelengths reflected from the surface of the stone is a very short interval of the spectrum. Therefore, their iridescent flashes are more like the colors of the rainbow, than a normal color gemstones that is created by a mixture of light rays with different wave length. Characteristic of black opal admixture of graphite and of iron and titanium contribute to the absorption of light waves, which partly explains their dark shades. For descriptions of iridescent color patches is used a number of special terms, such as: Arlekin (harlequin), flash (flash), and fleym (flame).

Unlike other varieties of quartz, opals are amorphous, not crystalline structure. They consist of tiny silica balls (globules), visible only under an electronic microscope. These spherical particles are formed by very slow evaporation of water, resulting in an increase in the concentration of silica in hydrothermal solution. Globules have approximately the same size and Packed in a strictly defined geometrical order, especially for the beautiful black opal.

The gaps between the spherical particles is filled with water, air and the same with amorphous silica. This kind of solidified silicon gel with a remarkably ordered structure is formed very slowly; the deposition rate is about 1 mm for 20 thousand years.
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All opals are divided into two categories: common and noble. The main feature of the latter is the play of colors on the surface or inside the stone. Perhaps the only exception is the fire opal, which also refer to the noble, although the color the game for them, and not typical.

The diameter of the globules noble black opal is 0.15 to 0.45 µm. Only particles of this size are able to create diffraction colors. The size of the globules also affects the color and character of iridescent flashes. Stones composed of balls smaller or larger diameter do not have the color game and they are called common opals. Black opal hardness: 6. Specific gravity: ~ 2.11 g/cm3. The index of refraction of 1.45. Water content: 5 to 10%.

Black opal found in the USA (Nevada), Indonesia, Mexico, Ethiopia. Discovered in Northern Kazakhstan (Akmola region), and also in pegmatites of Volhynia (Ukraine), where its dark color is due to organic impurities. But the only real source of noble black opal is Australia.
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About 95% of all noble opals supplied to the world jewellery market — Australian. Black opals are mined here since the second half of the XIX century. In the world jewelry market these beautiful stones appeared in the 1880s. The main deposits of opal are concentrated in the South-East mainland Australia, and are associated with deposits of Cretaceous in the Great Artesian basin. Black precious opal is mined only in two deposits: lightning ridge and Andamooka. The first is from 1901 and the second from 1930.

The most famous black opals: "Halley's Comet" (2020 CT.), "Queen of the night" (1520 CT.), Flamingo (800 CT.). The Smithsonian Museum (Washington) is treated with a cabochon black opal from Nevada — "Roebling", named after the American collector minerals V. Roebling (Roebling, Washington A. 1837 — 1926); the weight of this stone — 355,19 CT. And the most famous black opal is probably the "Fire of Troy" — a stone with bright red flowers, donated by Napoleon to his wife Josephine. Once this very beautiful gem is kept in the Museum of art history in Vienna, and then disappeared.

The color play of black opals usually occurs in veins, the thickness of which may range from a few hundredths of a millimeter to several centimeters.
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When such a layer is wide enough, the stone is treated in a plain cabochon. And if the iridescent layer too thin (as is often the case), the layer is carefully cut, sanded, and then glued on a substrate. The result is a so-called doublets. The surface of such products is usually almost flat.

If the layer thickness is even smaller (comparable to the thickness of a sheet of paper), the stone is polished on both sides, then glued on a substrate made of dark quartz rocks or black onyx and the front side is applied a transparent plate made of rock crystal or glass. Then this kind of three-layer "sandwich" are cut in cabochons. Such composite stones are called triplets. Their advantage is that a thin layer of precious opal is safely protected from scratches and other adverse effects.

Doublets and triplets may look no worse than these gems, but of course valued much lower. To hide the place of gluing the opal to the substrate, they are usually placed in a closed frame.

The front surface of the doublet is usually almost flat. Gemologists recognize them with a microscope, under which you can see the air bubbles in the glue that connects the thin plate of precious opal from the lower part of a composite stone. Sometimes doublets opal impregnated with resin, and the glue in them is mixed with crushed into dust by natural substance of the substrate. Such a qualitative composite stones even the experts identificeret with difficulty.

To amplify low-grade color play opal, they are sometimes impregnated with carbon particles or oil which then burns. These beautiful stones have the same refractive indices, but different for opals atypical granular structure that prevents high-quality polishing. Color flash in them and crayons scattered across the surface, generally resembling gold glitter French enamel. This black opal flashes larger and often have a whimsical ribbon-like form.

Sometimes porous light-colored opals are colored artificially to simulate black opal — impregnated with sugar syrup and concentrated sulfuric acid (sugar char).

The best quality imitation black opals are synthetic opals Pierre Gilson, obtained for the first time in 70 years. To distinguish them from natural gemstones can be very difficult. In the artificial stones are often dominated by red iridescent flashes, which are very rare in natural black opals. The stones of Gilson are generally more transparent and have a slightly lower specific gravity (vs 2.06 to 2.11 g/cm Australian black opals). They often have a columnar structure, particularly noticeable in the beads.

Areas of staining on their surface are distributed too evenly. Under the microscope the iridescent parts of the Gilson opals are distinguishable so-called "lizard skin" — the edges of the color spots are too pronounced, which never happens in these opals.
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Like many other synthetic jewelry materials, stones Gilson is often too beautiful to be real.

As a low-cost simulation can be used Slocum stones made of special glass. The refractive index and density of any glass imitations are always higher than real opals. The specific gravity of the stones Slocum — about 2.5; and a refraction index of about 1.5.

Plastic imitation can be very similar to real stones, but they are almost twice lighter natural opals. Their specific gravity: 1.1 to 1.2 g/cm3 (opals ~ 2,1). Can be distinguished and index of refraction (~ 1,49), which is slightly higher than the natural opals (~ 1,45).

Unfortunately, the beautiful black opals are far from perfect. It is inherent characteristic of any opals disadvantages: the fragility and relatively low hardness. These stones are rarely used in rings, where they can be easily damaged. In addition, they are very susceptible to moisture and drastic temperature changes, from which they will soon "age", covered with cracks. Sometimes opal to crack spontaneously; and it may occur immediately after treatment, and a few years after her.

However, any, even very significant disadvantages can not affect the popularity of these stones. Their gorgeous color and incredible, pure spectral color the game more than make up for any shortcomings. Noble black opals are truly one of the most beautiful gemstones, if not the most beautiful.