Black opal

Black opal is precious opal whose play-of-colour appears against a background ranging from dark grey to bluish black. The dark background makes the flashes of red, green and blue look far more vivid, which is why black opal is considered the rarest and usually the most valuable kind of opal. Most of the world's black opal comes from Lightning Ridge in the Australian state of New South Wales; dark-bodied opal is also found in Nevada in the USA, in Ethiopia and in Honduras. Because the look of black opal is often imitated with glass, plastic, layered stones and darkening treatments, it calls for care.

Jewellery: it is mostly cut as a cabochon (a dome) and set in rings, necklaces and earrings. Being softer than quartz, it needs protective settings and careful wear. Collections and museums: large, bright stones are displayed in museums; the 2,585-carat uncut “Roebling opal” at the Smithsonian came from Nevada. Science: the opalised fossils of Lightning Ridge are an important source for studying Australia's Cretaceous life. Symbol: the state of Nevada chose the black fire opal of Virgin Valley as its official state precious gemstone.

It has a hardness of 5.5–6.5 on the Mohs scale. It can scratch glass; a steel knife scratches it only with difficulty. Its specific gravity is 1.9–2.3 g/cm³.

Its best-known localities: Lightning Ridge, New South Wales, Australia; Mintabie, South Australia; Virgin Valley, Humboldt County, Nevada, USA; Stayish mine, near Gashena, Wollo, Ethiopia; Erandique, Lempira, Honduras.

As the stone moves, flashes of colour change and shift across a dark background. The background is dark grey, bluish black or black; a black stone without play-of-colour is not precious opal. Look at the side of the stone: layers glued together along a straight line give away a doublet or triplet, in which a thin slice of opal sits on a dark backing. If a loupe shows black stains or fine black dots on the surface or in cracks, the stone may have been darkened with smoke, dye, or sugar and acid. In natural black opal the dark colour is spread evenly through the stone. Doublets and triplets: a thin layer of precious opal is glued onto a black backing to give the look of black opal; triplets add a clear cap on top. They must be disclosed at sale.

On the “street to diamond” value ladder it sits on step 7 of nine: Precious stone. The value of black opal depends above all on the brightness and vividness of its play-of-colour; since a dark background shows those colours off most strongly, value generally rises as the background darkens. The range of colours (traditionally red is the most sought after), the beauty of the pattern, the thickness and size of the stone and freedom from cracks also count. Doublets, triplets, sugar-acid or smoke darkening and dyeing lower value greatly and must always be disclosed. Origin matters too: Lightning Ridge black opal is the best-known and most sought-after source.

According to Kunz in 1913, while some regarded opal as unlucky, black opal was considered an exceptionally lucky stone. There is no scientific evidence that black opal brings luck; like the belief that opal is unlucky, it is a popular superstition. The stone's colours come from tiny silica spheres inside it diffracting light; the dark background simply sets those colours off. This section reports beliefs; it is not health information. There is no scientific evidence for these claims and no stone replaces medical treatment.