Black opal: the chemistry, geology, history and how to spot a real stone

Why one stone shows the whole rainbow

Pick up a black opal and turn it under a lamp. Flashes of red, green and blue chase across the dark field like sparks on a night sky. There is no dye inside: the colour is born from pure physics of light. That puzzled people for thousands of years, and it still makes the stone one of the most recognisable in all of jewellery.

A black opal is not a "black stone with glitter". It is a variety of opal with a dark body on which the play of colour (the term is opalescence) reads with unusual contrast. Below we go through it properly: what the stone is made of, how it forms, where it is mined, how it was used through history, what the varieties are, how to tell a real one from a fake, and how to care for it. We will touch on symbolism too, but honestly: where there is tradition and where there is a verifiable fact.

The chemistry and physics of black opal

Composition and structure

Opal is a hydrated, amorphous silica. The formula is written as SiO₂·nH₂O: silicon dioxide with a variable amount of bound water. The water content is usually between 3 and 10 percent, sometimes more. That is what separates opal from quartz, which has the same chemical base but a strict crystal lattice.

Opal is amorphous, meaning it has no regular crystal structure and no crystal system of its own. Under an electron microscope you can see that it is built from tiny silica spheres of roughly equal size, packed into a more or less regular three-dimensional array. These spheres measure between 150 and 400 nanometres. It is precisely the regularity of that packing that produces the colour.

Mineralogists divide opal into several structural types. Precious opal (the kind that plays with colour) is classed as opal-AG and opal-AN, where the packing of spheres is sufficiently ordered. Common opal with no play of colour (opal-CT, opal-C) is far more frequent and counts for little in jewellery.

Where the rainbow comes from

The play of colour is the diffraction and interference of light on the regular grid of silica spheres. When white light passes between the ordered spheres, different wavelengths are deflected at different angles, and what reaches the eye is a spectrum already split into colours. The principle is the same as in a diffraction grating.

Which colour you see depends on the spacing between the spheres. Small spheres in tight packing give the cool part of the spectrum, blue and violet. Larger spheres give warm colours, all the way to red. Red play is therefore the rarest of all: it needs the largest, most evenly laid spheres, and that combination is scarce in nature.

The body colour, the stone's own tone, is no longer diffraction but absorption. In a black opal the dark body comes from finely scattered impurities: iron oxides and sulphides, and carbonaceous matter. The dark backdrop absorbs the light that did not go into diffraction, so the colour flashes read more brightly against it than against a milky-white body. The same physics of play works in the pale varieties, discussed in the piece on the rainbow opal, only there the body is light and the contrast is softer.

Hardness, density, optics

On the Mohs scale opal sits around 5.5 to 6.5. That is noticeably softer than quartz (7), let alone sapphire or diamond. The stone is relatively brittle and sensitive to knocks and scratches, which has a direct bearing on how to wear it (more on that in the care section below).

Opal's density is low for a gemstone, roughly 1.98 to 2.25 g/cm³. The water and the porosity of the structure account for that. The refractive index is also low, around 1.37 to 1.47. Opal is amorphous and optically isotropic, so it has no birefringence and no pleochroism: the play of colour delivers all of its optical drama, rather than dispersion in the classic sense, as in a diamond.

How opal forms in nature

Most gemstones crystallise from a melt or from hot solutions. Opal takes a different route: it precipitates from silica-rich waters at low temperatures, right near the surface of the earth.

Rainwater seeps through silica-rich rocks, dissolves some of the silica and carries it downward. In cracks, voids and cavities the solution gathers and gradually loses its water. The silica settles out as microscopic spheres, and if they manage to lie down evenly enough, a precious opal with a play of colour is formed. If the packing is disorderly, the result is common opal with no play. The process is slow and runs for millions of years.

Opal often fills the void left by something that came before: cracks, the pores of the rock, sometimes the cavities of shells, bones or wood. That is how opalised fossils arise, where the form of an organism is preserved while its substance has been wholly replaced by opal. Australia is known for finds of exactly this kind.

Geology and deposits

The world's main supplier of precious opal is Australia, and it is Australia that yields almost all of the gem-quality black opal. The opal fields formed in the sedimentary rocks of an ancient inland basin, once a shallow sea and later a long-weathered landmass.

The key points on the map:

• Lightning Ridge (New South Wales) is the chief source of dark and black opal. Most stones with a truly dark body come from here.
• Coober Pedy (South Australia) is the largest district for light opal. The town is famous for the fact that many of its houses and shops are dug underground to escape the heat.
• Andamooka and Mintabie are other Australian fields with their own types of opal.

Outside Australia opal is also mined, but mostly the light or semi-transparent kind. Ethiopia has brought a great deal of so-called Welo opal from volcanic rocks to the market. Mexico is known for fire opal with an orange-red body (that is a separate story, covered in the piece on the fire opal). Opal is found in Brazil, Peru and the United States. But the dark body that gives a true black opal remains largely an Australian trait.

The history of black opal

Antiquity and the name

The word itself goes back to the Sanskrit "upala" (stone, gem) and, through the Greek "opallios", came into Latin as "opalus". Opal was known in the ancient Mediterranean. The Roman writer Pliny the Elder, in his "Natural History", marvelled at opal and described it as a stone that combined the fire of a ruby, the brilliance of an amethyst and the green of an emerald all at once. That is perhaps the earliest clear description of the play of colour in literature.

In antiquity and the Middle Ages, "opal" mostly meant the light, playing stones brought, by later understanding, from the East. The dark Australian opal was not yet known to Europe at that time.

The stone's reputation

Opal has a complicated reputation. In the Middle Ages and later it was at times prized as a stone of luck and keen sight, at times feared. The stubborn superstition that opal supposedly brings misfortune took particular hold in nineteenth-century Europe. Historians of jewellery usually link it to the literature of the period and to the fact that the soft stone was easily damaged and cracked on its owners. The belief has no factual basis whatsoever; it is pure cultural history.

The turning point in perception came when Queen Victoria in Britain openly wore opals and gave them as gifts. Royal taste did much to lift the stone's bad name and to support the fashion for it.

The Australian finds

The industrial history of black opal begins in Australia at the turn of the nineteenth and twentieth centuries. The opal fields of New South Wales, and Lightning Ridge above all, yielded stones with a dark body and bright play never seen before. From that moment the black opal became the rare and costly stone it is known as today. Australia secured its standing as the opal country and later made opal its national gemstone.

Types and varieties of opal

Opal is classified first of all by the tone of its body and by the character of its play.

By body:

• Black opal has a dark body, from dark grey to near-black. The most valuable group, because on a dark field the play of colour reads with the greatest contrast.
• Dark (semi-black) opal has a grey body, an intermediate option.
• Light and white opal has a milky, pale body. The most common.
• Crystal opal is transparent or translucent, with the play visible as if in volume.
• Boulder opal is opal in thin seams on the parent rock (ironstone). The stone is cut together with its backing of natural rock, and the dark rock strengthens the play. It is a single natural material, not a glued assembly.
• Fire opal has an orange-red body, with a play of colour that may be present or absent.

By pattern of play:

• Pinfire has small, frequent sparks across the whole stone.
• Flash has large patches of colour that flare up when the stone is turned.
• Harlequin (mosaic) has large, angular blocks of colour laid like a mosaic. The rarest and most valuable pattern.

The quality of black opal on the market is usually described by the tone of the body (on a scale from light to black), by the brightness of the play, by the dominant colour (red is rarer and dearer, blue and green more ordinary), by the pattern and by the clarity.

Solid opal, doublets, triplets and synthetics

What is sold as a solid stone is often a composite. The difference matters both for the price and for the durability.

• Solid opal is one natural stone from the body to the surface. The most valuable option.
• Doublet is a thin slice of opal glued onto a dark base (often a black material or potch, colourless opal). The dark backing imitates a black body. Cheaper than a solid stone, but the join shows along the edge.
• Triplet is the same as a doublet, plus a protective dome of clear material (quartz, glass) on top. It often gives an unnaturally mirror-like shine.
• Synthetic opal is grown in a laboratory. It has a play of colour, but under magnification you often see an overly regular "snakeskin" or columnar structure to the colour zones.
• Imitations are glass, plastic and sintered materials. The play looks even and monotonous.

How to tell a real opal

A few practical signs:

• Look at the edge of the stone from the side. In a doublet and a triplet you see a straight glue line and a sharp boundary between layers. In a solid opal the colour transition is smooth and natural.
• The underside. A solid opal usually has an uneven natural base, while a doublet has a flat bottom, ready for gluing.
• The pattern of play. In a synthetic, the colour zones often fall into regular columns or scales, visible under a loupe when tilted. In a natural opal the pattern is irregular.
• The shine. A triplet under a glass dome shines like a polished lens, even and cold. A natural opal gives a softer glow.
• The evenness of the body. A perfectly even body without the slightest variation is suspect: a natural stone is always a little uneven.

The most reliable route for an expensive purchase is a report from a gemmological laboratory. It confirms that the stone is natural and solid, and states whether any treatment has been applied.

Treatment

The dark body is sometimes enhanced artificially: by impregnation with sugar followed by acid treatment (the stone carbonises and darkens) or by smoking. Such methods are lawful if they are disclosed, but a treated stone is worth less than a naturally dark opal and must be declared as treated.

Crazing: the main risk with opal

Opal has a feature that hard gemstones do not, and one buyers are rarely told about. The stone can develop a web of fine cracks, called crazing. The cause is water: opal holds it inside its structure, and when the moisture is lost quickly the body of the stone contracts unevenly, the surface becomes covered with a cobweb of cracks, and the play of colour goes out in those places.

Crazing is provoked by sudden drying, direct sun in a display case, a hot hairdryer, a radiator under the jewellery, or a flight in a dry cabin. Opal from certain deposits is more prone to it, which is why a mined stone is often left to settle for months before cutting: if it is going to crack, better it happens before the sale than on the owner's finger. Australian solid opal is one of the most stable in this respect, while Ethiopian hydrophane opal (which absorbs water and clouds over temporarily) behaves more capriciously.

What follows from this when buying. Examine the surface under bright side light and under a loupe: a cobweb of cracks that has already started shows as a thin net of lines. This is not a cutting defect, it is irreversible, and such a stone is not worth taking. With a reputable seller the opal has already gone through its natural settling by the time of sale. Haste here is on the buyer's side: a stone that has lain stable for a year will almost certainly stay that way.

The cut and how to judge a stone face-up

Opal is almost always cut as a cabochon, a smooth dome with no facets. The reason is not tradition but physics: the play of colour is a surface optical effect, and a faceted cut like a diamond's would add nothing to it while losing mass to the facets. A domed shape catches light from every angle and shows the shimmer at any turn.

The height of the dome and the thickness of the playing layer are money. In cheap stones the colour layer can be a fraction of a millimetre, and during repolishing it may simply come away. In a good solid opal the playing layer is thick and the dome is convex. A flat stone, almost like glass, should put you on guard: it is either a very thin layer or a composite doublet.

Opal is judged face-up, as it will be seen in the jewellery, not from the side. Things to watch for:

• Directionality of play. Turn the stone every way and tilt it. In a good opal the flashes live at any angle. In a weak one the colour is visible only from one position and dies from others; such stones are called directional and valued lower.
• Dead spots. Areas with no play, dull patches of the opal body. The fewer and the less noticeable they are, the dearer the stone.
• Brightness over colour. A dull red loses to a bright blue. First you look at the strength of the glow, and only then at the palette.
• Where the play sits. The shimmer should be right at the surface of the dome, not somewhere deep beneath a cloudy layer.

The shape is more often left freeform (oval, teardrop, an uneven outline) to keep the maximum of playing material: an even calibrated oval in an opal means that some of the precious colour went to waste for the sake of geometry.

Caring for black opal

The softness and porosity of opal dictate some simple rules.

Wearability. A hardness of 5.5 to 6.5 means the stone is easily scratched by sand and dust (which contain quartz at hardness 7) and is afraid of knocks. For a ring worn every day and knocked against everything, opal is a risky choice: better to save it for earrings, a pendant or an occasion ring. Boulder opal and a solid stone are tougher than a thin doublet.

Cleaning. Warm (not hot) water, mild soap, a soft cloth or brush. No ultrasonic cleaning and no steam: sharp swings and vibration produce micro-cracks. Give aggressive chemicals and solvents a miss too.

Water and heat. Take the opal off before a shower, a sauna, the pool and the sea. Sharp temperature swings and drying out are the most harmful things of all for the stone.

Storage. Keep opal away from hard stones to avoid scratches, in a soft pouch or a separate compartment. The advice to "store it in water" does not apply to precious Australian opal; there is no need to keep it constantly wet. Simply avoid extremely dry and hot air.

Setting. Once a year it is worth checking that the stone is held firmly in its setting, especially with rings.

Symbolism: what is tradition and what is fact

Opal has been credited with all sorts of things. In some traditions it is a stone of loyalty and hope, in others, on the contrary, a source of superstitious fear. The play of colour long gave reason to see it as a stone of change and many sides, since it literally never looks the same twice.

It is worth saying plainly: opal has no healing or "energy" properties, and there is no proven effect on health, sleep, anxiety or luck. All of that belongs to the realm of belief and cultural association, not to physics or medicine. The stone is beautiful in itself, and that is enough. If the symbolism of change appeals to you, treat it as a pleasant story rather than a property of the material.

What to wear black opal with

A black opal loves a dark backdrop around it, so the main rule is simple: give the stone a stage on which its shimmer will be the only bright spot. It is a piece that sets the tone for the whole look on its own, rather than topping up a dozen other details.

For everyday wear a ring or small studs with an opal work beautifully with plain knitwear, a grey or charcoal roll-neck, a classic white shirt. Black clothing is the ideal ally here: against it the shimmer flares strongest, and even a modest stone looks expensive. With jeans and a cashmere jumper an opal in silver or white gold suits, with no extra shine alongside.

For the office, choose a restrained form: a pendant on a fine chain over a shirt, or stud earrings. The opal draws the eye anyway, so keep the rest of the jewellery to a minimum; one noticeable piece is enough. A deep V-neckline calls for a mid-length pendant, so the stone sits just above the décolletage and catches the light as you move.

An evening out is the territory where opal comes fully into its own. Bare shoulders, smooth satin or velvet in deep shades (emerald, wine, navy) echo the colours inside the stone. Drop earrings or a large pendant are at home here, and you can do without a necklace so as not to compete with the opal.

By metal the logic runs like this: white gold and silver give maximum contrast and cool drama, while yellow and rose gold soften the stone and suit a warm colouring. If you already own several gold pieces, an opal in the same metal tone will pull the look together.

Who does black opal suit especially? Those who do not fear attention and prize things with character, who like one strong accent rather than a scatter of trifles. A parting word: wear one opal piece at a time and do not overload the hand with a clutter of rings next to it; the stone needs room to breathe light.

Opal and the metal of the setting

A cool metal suits a dark opal. White gold and silver give the strongest contrast: a pale setting emphasises the dark body and makes the colour flashes more visible. Yellow and rose gold soften the stone and lend it warmth, a choice that works well for classic and vintage forms. Silver is more affordable but oxidises and needs care; the standard fineness of jewellery silver is 925.

If you want a bright play of light but in a stone tougher for everyday wear, take a look at zircon in all its colours: it is harder than opal and copes more calmly with knocks and water.

Frequent questions about black opal

What is opal made of?

It is hydrated silica, SiO₂·nH₂O. Chemically the same base as quartz, but without the crystal lattice: opal is amorphous and contains from a few to ten or more percent water.

Why does opal shimmer in every colour?

The play of colour is the diffraction of light on the regular packing of microscopic silica spheres. The spacing between the spheres sets the colour: small spheres give blue, large ones give red. That is exactly why red play is rare.

How does black opal differ from white?

Only in the tone of the body. A black opal has a dark stone body, against which the play of colour reads with more contrast. The physics of the flashes themselves is identical.

Which opal is the most valuable?

A black opal with a bright, multicoloured pattern, especially if there is plenty of red in the play and the pattern is large (harlequin). The darker and more even the body, and the brighter the play, the dearer the stone.

Where is black opal mined?

Almost all gem-quality black opal comes from Australia, above all from Lightning Ridge. Light opal also comes from Ethiopia, Mexico and Brazil, but the dark body is an Australian speciality.

What is opal's hardness and can it be worn every day?

On the Mohs scale 5.5 to 6.5, a soft and brittle stone. For an everyday ring it is better to choose a harder stone, and to save the opal for earrings, a pendant or occasion pieces.

How do you tell a real opal from a doublet or triplet?

Look at the stone from the side: in composites you see a straight glue line and a sharp boundary between layers, while a triplet has an even glass dome on top with a cold shine. In a solid opal the transitions are smooth and the base is uneven. For an expensive purchase get a laboratory report.

How do you tell a natural opal from a synthetic?

Under a loupe, when tilted, a synthetic often shows overly regular colour zones: columns or scales. The natural pattern is irregular and the body a little uneven. A precise answer comes from a gemmological examination.

How do you clean opal?

Warm water and mild soap, a soft cloth. No ultrasound, steam, boiling water or solvents. Take the piece off before the pool, the sauna and the sea.

Is it true that opal brings misfortune?

It is a superstition, mostly European and mostly of the nineteenth century. It has no factual basis. The stone has no healing or "energy" properties either; that is the realm of belief, not of proven effects.

About Zevira: jewellery with black opal

In the Zevira catalogue black opal is a stone chosen one at a time: no two are alike, each with its own pattern of play and its own body tone. We look at the darkness of the body, the brightness and colour of the flashes, the clarity and the secure seating of the stone in its setting.

Every opal piece is checked for the stone, the setting and the finish of the metal. For a dark opal we more often choose a cool metal, white gold or silver, so the contrast works for the play of colour.

The short version

Black opal is a hydrated, amorphous silica, a soft (5.5 to 6.5 on the Mohs scale) and brittle stone whose famous play of colour is born not from paint but from the diffraction of light on ordered silica spheres. The dark body is created by iron and carbon impurities, and it is that body which makes the colour flashes so bright. Almost all gem-quality black opal is mined in Australia.

When buying, look at the wholeness of the stone and beware of doublets, triplets and synthetics, and for expensive pieces get a laboratory report. In care, the main thing is to protect the stone from knocks, ultrasound, heat and sharp swings in humidity. And all the mysticism around opal is a pretty piece of cultural history, no more than that.

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