Black Opal

Silica · also called Lightning Ridge opal, Dark opal

Black opal is precious opal with a dark body tone, graded N1 to N4 on the Australian scale, against which its play-of-colour reads at full strength. Amorphous hydrated silica rather than a crystal, hard 5½ to 6½, it comes almost entirely from the Lightning Ridge field in New South Wales.

What is black opal?

Black opal is precious opal whose body tone is dark — N1 to N4 on the Australian scale — so that the play-of-colour is seen against an unlit background rather than a white one. It is a hydrated silica mineraloid rather than a crystalline mineral, and Lightning Ridge in New South Wales has supplied nearly all of it.

The name misleads more buyers than any other in the opal trade. Black opal is not black in the way that jet or black spinel is black; the term describes the ground against which the colour is seen, not the colour itself. A stone of N1 body tone with a brilliant red flash is black opal, and so is a sombre N4 with a weak green shimmer, which is why the grade on its own settles very little. Body tone is assessed face-up against a standard set of nine tones, N1 the darkest and N9 the whitest, with N5 and N6 usually described as semi-black or dark opal and N7 to N9 as light opal. Crystal opal, which is transparent to semi-transparent, is judged on its own terms and is a different proposition again.

What gives black opal its colour?

Two separate things, and confusing them is the root of most mistakes. The play-of-colour is diffraction, produced by an ordered three-dimensional array of silica spheres roughly 150 to 400 nanometres across. The dark body tone is something else entirely: it comes from carbon and iron oxides in the common opal, or potch, beneath the colour.

The diffraction model was established at CSIRO in the 1960s, when electron microscopy showed that precious opal consists of uniform spheres in a regular lattice and common opal the same chemistry disordered. Sphere size determines which wavelengths are returned: small spheres give blue and violet, larger ones green, and the largest red. Because the largest well-ordered spheres are the hardest to form, red is the scarcest colour in opal and blue the most abundant, an ordering of rarity that runs through the whole price structure. The body tone contributes nothing to the diffraction itself; it removes the scattered white light that would otherwise dilute it, which is why the same colour bar looks incomparably stronger on black potch than on white.

How is black opal identified?

A spot refractive index between about 1.42 and 1.47, a specific gravity between roughly 1.98 and 2.25, no cleavage, a conchoidal fracture and a singly refractive reading under the polariscope, commonly with anomalous strain birefringence. Nothing else of this appearance shares that combination, so identifying solid opal as opal is straightforward.

The difficulty is never whether the material is opal, but whether the stone is solid and whether the dark body is natural. Assembled stones are the first question: a doublet is a slice of colour bar cemented to a dark backing of potch, ironstone or black glass, and a triplet adds a clear cap of quartz or glass on top. Both are caught by looking at the stone from the side rather than the face, where the join appears as a straight line, usually above a flat and unnaturally regular base. Manufactured opal betrays itself under magnification by a columnar structure that breaks the colour patches into a snakeskin mosaic, and plastic imitations read far too low on the refractometer.

The distinguishing feature: a colour bar on potch

What belongs to black opal and to nothing else is its layered construction — a thin bar of play-of-colour sitting on a dark base of common opal, the colour layer rarely more than a few millimetres thick and often very much less. The cutter's whole task is to expose that bar and keep as much of it as possible.

This explains most of what looks eccentric about the trade. Black opals are cut free-form, in irregular outlines with domed or undulating tops, because grinding to a symmetrical oval would mean grinding good colour away. They are cut as cabochons because facets would do nothing for a stone whose colour is diffracted rather than refracted. Lightning Ridge material is distinctive in habit too: much of it occurs as nobbies, rounded nodules of opal detached from the enclosing claystone, as against the flat seam opal that dominates the South Australian fields. And because a diffraction grating works in particular directions, many black opals are directional, brilliant across one axis and dead across another. A stone that must be held at one angle to show what it can do is worth materially less than one that fires from everywhere.

Is black opal treated?

Australian black opal is, as a rule, untreated: cut, polished, and nothing further. The treatments to know about are those applied to paler, porous material to make it resemble black opal, and the assembled stones sold alongside it.

Three processes are in circulation. Smoke treatment wraps porous opal in paper and burns it, depositing carbon in the surface pores and leaving a dark skin thin enough to reveal itself at a chip or an unpolished edge. Sugar treatment soaks the stone in a sugar solution and then carbonises it with sulphuric acid, blackening it more deeply. Dyeing reaches a similar end with colourants. All three depend on porosity, so they work on hydrophane opal, notably some Ethiopian material, and on Andamooka matrix, and they do not work on solid Lightning Ridge opal, which is not porous. Each must be disclosed, as must doublet and triplet construction. A dark opal offered without a stated origin and without the word "solid" deserves scepticism.

Where does black opal come from?

Overwhelmingly from Lightning Ridge in northern New South Wales, where opal occurs in the Cretaceous claystones of the Griman Creek Formation along the eastern margin of the Great Artesian Basin. The surrounding fields — Coocoran, Grawin, Glengarry, Sheepyard — are worked by small operators on individual claims rather than by any large mining company.

Other Australian fields produce opal but not, in commercial quantity, black opal. Coober Pedy in South Australia is the classic source of white and light opal, Andamooka of light opal and of the matrix material mentioned above, and Mintabie produced dark crystal opal until that field closed at the end of the 2010s. Outside Australia, some Ethiopian material from the Stayish area of Wollo has a genuinely dark natural body, Java produces dark stones, and Honduras a black matrix opal in which colour is dispersed through the host rock. None of these leads the market. Lightning Ridge is also the source of the opalised Cretaceous fossils — bone, shell and wood replaced by precious opal — that occupy their own corner of the specimens market.

Where the name comes from

Opal descends from the Sanskrit upala, a stone or precious stone, through the Greek opallios and the Latin opalus. Black is a twentieth-century trade addition describing body tone rather than the stone's appearance. The material was found at Lightning Ridge around the turn of the twentieth century and mined there commercially from the first years of the 1900s.

The early history is instructive, because the market resisted the stone. Dark opal was unfamiliar, European dealers suspected it of having been doctored, and Charlie Nettleton, credited with the first serious working of the field, is reported to have had real difficulty selling what he brought out. Opal in general carried a superstition of ill luck that gemmological writers have long traced to Walter Scott's novel Anne of Geierstein of 1829, in which an opal's fortunes track its owner's; the association had no earlier basis. Black opal outlived both obstacles to become the most valuable form of the species, and opal is the national gemstone of Australia.

What sizes are available?

Black opal is a thin material, and thickness rather than outline usually decides what can be cut from a parcel. Most finished stones fall between roughly one and five carats. A fine stone above ten carats is uncommon, and the largest are individual pieces that trade on their own merits rather than at any per-carat rate.

The relation between weight and value is looser here than for a faceted stone, and it fails in both directions. A cabochon left with a thick potch backing weighs more without showing more, so weight can be paid for that is never seen; conversely a broad, shallow stone with a large face and modest weight may be the better buy. Face-up area, the thickness of the colour bar and the evenness of the dome are the things to measure. Matched pairs and suites are difficult and priced accordingly, since body tone, brightness, pattern and directionality all have to agree across stones that were never cut to a standard.

How much is black opal worth?

In order of weight: brightness first, then body tone, then the colour range within the play, then pattern, then face-up size, then freedom from faults. A brilliant N5 will outsell a dull N1, which is the single most useful thing to know when reading any opal price list.

Brightness is judged face-up in diffuse daylight, graded by the trade on a scale running from brilliant to subdued, and it is the property most often overstated in photographs taken under a point source. Body tone comes next, N1 to N4 defining black opal proper. Within the play, red is the scarcest colour and carries the largest premium, followed by orange, then green, with blue the commonest; a stone showing the full range is worth more than one of a single colour. Pattern then modifies everything: harlequin, a mosaic of broad angular patches of roughly even size, is the rarest and dearest, with flagstone, ribbon, rolling flash and floral above small, even pinfire. Faults subtract — sand or potch lines crossing the face, a window of dead colour, crazing, or a bar so thin that it would not survive a repolish.

Is black opal durable enough to wear?

At 5½ to 6½ on the Mohs scale opal is softer than the quartz dust in ordinary household grit, and although it has no cleavage it is brittle and fractures conchoidally. It is sensitive to heat, to sudden changes of temperature and to knocks. Black opal is a stone to protect rather than one to forget about.

A bezel or rub-over setting is the sensible mount for a ring, and a pendant or a pair of earrings will always give the stone an easier life than a ring worn daily. Never clean opal in an ultrasonic tank or with steam, and keep it away from hot water, solvents, oils and acids; a soft cloth, lukewarm water and mild soap are enough. The crazing that opal is feared for — a fine network of surface cracks caused by water loss — is far more a hydrophane problem than an Australian one, and solid Lightning Ridge opal from a settled parcel is a stable material. The old advice to store opal in water is unnecessary for solid Australian stones and actively harmful to doublets and triplets, whose cement can be undermined by moisture.

How to buy black opal without being caught out

Four questions answer nearly everything: is the stone solid, what is its body tone, how bright is it face-up in ordinary light, and what does it do when it is turned. Ask for the answers in writing, and look at the stone from the side before looking at the face.

The side view settles the assembled-stone question, since a doublet or triplet shows a straight cemented join and a flat base where a solid stone shows an irregular natural back, often with potch or ironstone still attached. Judge brightness under diffuse daylight rather than the spotlight used in most photographs and videos, and rotate the stone through a full turn to test for directionality and for windows of dead colour. Ask for the body tone as a stated N number and for the origin as a stated field. Examine the surface under magnification for crazing and for sand inclusions. Where the price justifies it, request a report from a recognised laboratory such as GIA or Gem-A confirming that the stone is natural, solid and untreated, or arrange independent testing through our services. For a comparison with another material coloured by interference rather than absorption, see ammolite; for the wider silica family to which opal belongs, see agate and the rest of the stones library.

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