Opal is the only major gemstone judged mainly on a trick of light. A diamond's value comes down to how well it's cut from a fixed set of properties; a sapphire's comes down to how saturated its colour is. An opal's comes down to play-of-colour, the shifting flashes that appear and vanish as the stone turns in your hand, caused by microscopic spheres of silica diffracting light inside the stone, with no pigment involved at all. Get that moving, nothing-to-hold-onto quality right, and you've bought the single most interesting stone in this house's whole birthstone cluster. Get the construction wrong, and you've bought a thin wafer of opal glued to a dark backing, which isn't a scandal, just a different purchase than the one you thought you were making.
In Brief: opal quality is judged first on play-of-colour (the brightness and spread of the flashes), then body tone (the background colour it plays against, darker generally means more valuable), then pattern and construction. A solid opal is cut entirely from natural material; a doublet glues a thin opal slice to a dark backing and typically sells for 15 to 35 per cent of an equivalent solid's price; a triplet adds a protective dome on top and sells for less again. Australia supplies roughly 95 per cent of the world's precious opal, and solid Australian opal does not absorb water; Ethiopian opal is often hydrophane, meaning porous and water-absorbing, and is sometimes smoke- or sugar-treated to darken its body tone, which must be disclosed. Expect roughly £10–£300 per carat for good white or crystal opal, and considerably more for fine black opal from Lightning Ridge.
Play-of-colour comes first, and it's not the same as colour
Play-of-colour is the flashes themselves: whether they're bright or dull, whether they cover the whole stone or sit in one patch, and whether the colour spread runs across the full spectrum or stops at blue and green. A broad, even spread of colour that holds up as you tilt the stone is worth more than a single flash of colour from one angle, in the same way an evenly cut sapphire beats one with a dead window at the centre. Red is the rarest flash colour and commands the strongest premium; blue and green are the most common.
This is the one factor that can't be photographed honestly. A still image catches opal at a single angle, and the whole point of the stone is that it isn't static. Buy opal in person if you possibly can, and move it under more than one light source before you commit.
Body tone is the stage, not the show
Body tone is the background colour the play-of-colour performs against, graded on a scale from N1, the darkest black, to N9, milky white. It isn't itself the source of value, but it changes how much the play-of-colour reads: the same flash of red or green shows up far more vividly against a black background than a pale one, which is why black opal (N1 to N4) commands such a steep premium over white opal of otherwise comparable quality. Dark opal (N5 to N6) sits between the two, offering much of black opal's contrast at a meaningfully lower price.
Construction: solid, doublet, and triplet aren't a hierarchy of honesty
This is the area that trips up more opal buyers than any other, and it shouldn't, because none of the three constructions is inherently dishonest. A solid opal is cut entirely from natural material, top to base. A doublet is a thin slice of precious opal glued to a dark backing, usually ironstone, black potch (common opal with no play-of-colour) or glass, which lets a cutter use opal too thin to stand on its own and also deepens the apparent body tone for free. A triplet adds a domed cap of clear quartz or glass on top of a doublet, both protecting the thin opal layer underneath and magnifying its play-of-colour.
All three are legitimate categories, provided they're sold as what they are. CIBJO, the international jewellery confederation, and trade bodies including the Opal Association require doublets and triplets to be described explicitly as "opal doublet" and "opal triplet," never as plain opal. The honest reason to buy a doublet or triplet is price: doublets typically sell for 15 to 35 per cent of what a comparable solid opal would cost, occasionally up to half for the finest examples, and triplets far less again, since they use even less actual opal. The honest reason to avoid them is durability: the glued layer is a weak point, and a triplet in particular is not something to wear in a ring that takes daily knocks.
Checking is simple before you buy anything described as solid. Look at the stone side-on. A visible seam, a flat line where colour abruptly changes, or a domed clear layer on top are all signs of assembled construction. A solid opal shows consistent material all the way through.
Treatment: smoke, sugar, and the honest reason they exist
Beyond assembly, some opal is chemically treated to darken its body tone and make the play-of-colour read more strongly, essentially imitating the contrast of black opal at a fraction of the price. Smoke treatment wraps the stone in paper, which is then smouldered so that fine soot particles enter the opal's pores and darken it. Sugar-acid treatment soaks the stone in warm sugar solution, then sulfuric acid, which carbonises the sugar inside the pores to the same darkening effect. Both rely on the same porosity that makes Ethiopian opal hydrophane, covered below, and both are detectable under laboratory examination.
Treated opal is legitimate and often attractively priced, exactly like heat-treated ruby or sapphire, provided it's disclosed. An undisclosed smoke-treated stone sold as natural black opal is the opal trade's equivalent of an undisclosed composite ruby: not illegal everywhere, but a straightforward case of paying black-opal prices for white-opal material.
Where an opal is from changes what it needs from you
Origin matters for opal less as a prestige marker and more as a practical warning, because the major sources behave differently once they're in daily wear.
Australia supplies roughly 95 per cent of the world's precious opal and is the standard the whole trade is measured against. Lightning Ridge in New South Wales is the near-exclusive source of fine black opal; Coober Pedy and Andamooka in South Australia produce most white and crystal opal; Queensland's Winton, Quilpie and Yowah fields produce boulder opal, where a thin seam of precious opal remains attached to its host ironstone rather than being cut free. Solid Australian opal is not hydrophane, meaning it doesn't absorb water through its structure, which makes it considerably more forgiving to wear day to day.
Ethiopia, mainly the Welo region, has become a significant and much more affordable source since a major find in 2008, with vivid, often large-patterned play-of-colour. The complication is that much Ethiopian opal is hydrophane: it absorbs water and other liquids through interconnected pores, sometimes gaining up to 15 per cent of its weight in water, and can turn temporarily translucent or change appearance when wet before drying back out over days or weeks. This isn't necessarily damage, but it is a real difference in how the stone needs to be worn and cared for, and a seller who doesn't mention it when selling Ethiopian material isn't doing their job.
Mexico produces fire opal, transparent to translucent stones in vivid orange and red body colours, sometimes with play-of-colour and sometimes valued for the body colour alone.
Caring for a stone that's partly water
Opal contains somewhere between 3 and 21 per cent water by weight, usually 6 to 10, bound into its structure, and that water content is the reason opal needs different care from almost anything else in a jewellery box. Dry it out too fast, through extreme heat, direct sun for extended periods, or storage in a very dry environment such as a safe or safety deposit box, and the stone can develop crazing: a network of fine internal cracks that permanently changes its appearance and weakens it. Ultrasonic and steam cleaners are a hard no for the same reason, since the heat and vibration can crack the stone outright.
The practical rules: avoid rapid temperature swings, keep opal away from prolonged direct sunlight and very dry storage, and clean it with nothing stronger than mild soap, lukewarm water and a soft cloth. Hydrophane Ethiopian opal needs an extra layer of care on top of this: take it off before swimming, showering, exercising or applying skincare, since it will absorb whatever it's sitting in.
Certification: when to ask, and what it actually tells you
For a significant purchase, a report from a major lab such as GIA will confirm construction (solid, doublet or triplet) and identify any treatment; origin is harder to establish for opal than for corundum, so don't expect it as standard. Ask plainly: is this solid, and if not, is it described as a doublet or triplet on the receipt? Has it been treated to darken the body tone? Is it hydrophane, and if so, what does that mean for how I should wear and store it? A seller who can answer all three without hesitation is one worth buying from.
Quick Reference
| Type | Body tone | Construction notes | Roughly per carat |
|---|---|---|---|
| White / light opal | N7–N9 | Usually solid | £10–£300 |
| Crystal opal | Variable, transparent | Usually solid | £30–£300 |
| Boulder opal | Variable, attached to host rock | Solid, priced on total weight including host | £30–£800 |
| Dark opal | N5–N6 | Usually solid | £100–£1,500 |
| Black opal (Lightning Ridge) | N1–N4 | Solid; the benchmark for value | £500–£15,000+ |
| Ethiopian opal (Welo) | Variable, often hydrophane | Solid; smoke/sugar treatment common and must be disclosed | £10–£75 for good material |
| Doublet | As backing material dictates | Thin opal glued to dark backing | 15–35% of equivalent solid |
| Triplet | As backing material dictates | Doublet plus protective dome | Far less than a doublet |
Figures are indicative and will move with market conditions and exceptional individual stones. Famous black opals, such as the Aurora Australis, are valued well outside this table. For how opal got its reputation for bad luck, see our Opal Curse story. For the companion stones in this autumn's cluster, our sapphire and ruby buying guides cover the same ground for corundum.
FAQ
Is a doublet or triplet opal a fake? No. Both are genuine, precious opal, just assembled with a backing (and, for triplets, a protective dome) rather than cut as a single solid piece. They're legitimate provided they're sold honestly as doublets or triplets, not as solid opal, and they cost meaningfully less than an equivalent solid stone, which is their real advantage.
Why is black opal so much more expensive than white opal? Black opal's dark body tone makes its play-of-colour read far more vividly, and it comes almost exclusively from Lightning Ridge in Australia, a tightly limited source. The combination of superior visual contrast and scarcity drives the price gap, which can run from tens of pounds a carat for white opal to many thousands for fine black.
Can opal get wet? Solid Australian opal tolerates everyday contact with water fine, though it shouldn't be soaked or exposed to extreme temperature swings. Hydrophane opal, common in Ethiopian material, actively absorbs water through its pores and should be kept away from swimming, showering and skincare products, since it will change appearance when wet and the long-term effects of repeated soaking aren't fully predictable.
What is smoke-treated opal? A treatment, most often applied to porous Ethiopian opal, where soot particles from smouldering paper enter the stone's pores and darken its body tone, making the play-of-colour appear stronger and imitating the contrast of black opal at a much lower price. It's a legitimate treatment when disclosed, and a problem only when it's sold as untreated natural black opal.
Why does opal crack? Opal contains water bound into its silica structure, and if that water is lost too quickly, through heat, prolonged sun exposure or very dry storage, the stone can develop crazing, a network of fine internal cracks. Keeping opal away from extreme or rapid temperature and humidity changes is the main way to prevent it.
Sources
- Opal, Wikipedia
- Opal Gems: Value, Price, and Jewelry Information, IGS
- Australian Opals: An Introduction, IGS
- Opal Mining in Coober Pedy, IGS
- Opal Value & Price per Carat Explained, Cindy Xu Jewelry
- Opal Price Guide (GBP), Gem Mine
- Why Australian Opal Is the Best in the World, Opal Galaxy
- Ethiopian Opal vs Australian Opal, The Bead Traders
- Educational Opal Facts List, Australian Opal Direct
- Ethiopian Opals, Geology.com
- Ethiopian Opal: The Complete Guide
- Natural Black Opal Identification, Black Opal Shop
