Opal: Why It Plays With Color, and What It Asks in Return
The Question Nobody Could Answer Until 1964
For as long as people had worn opal, nobody knew why it did what it does. Every other gemstone owes its color to chemistry: chromium makes ruby red, iron makes aquamarine blue. Opal is essentially colorless silica and water, and yet it throws red, green, and violet across a room. The explanation arrived only when Australian researchers put an opal under an electron microscope in 1964 and found what no optical instrument could show: microscopic spheres of silica, all nearly the same size, stacked in a regular three-dimensional grid.
Opal does not contain color. It builds color out of light, the way a compact disc does, and the size of those spheres decides which colors appear. Everything that makes opal difficult, delicate, expensive, and worth the trouble follows from that one structural fact.
What Opal Actually Is
Opal is hydrated silica, chemically SiO2·nH2O. It has no crystal structure, which makes it a mineraloid rather than a mineral, and puts it in the same category as amber and natural glass rather than alongside sapphire or garnet.
Water content ranges from about three to twenty-one percent, most commonly between six and ten. That water is not decoration. It sits inside the silica structure, and whether it stays there governs the stone's entire future.
The practical figures follow from the same structure. Opal rates 5.5 to 6.5 on the Mohs scale, softer than quartz and therefore vulnerable to ordinary household dust, which is largely quartz. Its specific gravity of 1.88 to 2.50 is the lowest of any gemstone in common use, so an opal feels noticeably light for its size; our density guide places it at the bottom of the scale. Refractive index runs 1.37 to 1.52, low enough that opal shows almost no brilliance in the conventional sense. It has no need of any.
Why Opal Plays With Color
The silica spheres inside precious opal measure roughly 150 to 400 nanometres across, and where they are uniform in size and stacked in an orderly grid, they diffract light. White light entering the stone is split, and the wavelength that emerges depends on sphere diameter and viewing angle.
This produces the single most useful fact a buyer can know about opal pricing:
- Spheres near 150 nanometres diffract the short wavelengths and produce violet and blue.
- Spheres around 250 nanometres produce green and yellow.
- Spheres near 350 nanometres and above produce orange and red.
Large spheres are harder for nature to grow uniformly than small ones, so red play-of-color is scarce and blue is common. An opal that shows strong red commands a substantial premium over an otherwise identical stone showing only blue, and a stone showing the full spectrum, red included, sits at the top of the market. This is not a matter of taste. It is a matter of how difficult the arrangement was to build.
Where the spheres are irregular in size or randomly packed, no diffraction occurs and the material is common opal, called potch in the trade, which has no play-of-color at all. The difference between a stone worth thousands and one worth nothing can be a matter of nanometres.
The Types, and Which Ones We Carry
Precious opal is any opal showing play-of-color. Everything below is a subdivision of it, except where noted.
Black opal has a dark body tone that makes the diffracted colors read more vividly against it. Lightning Ridge in New South Wales is the classic source, and fine black opal reaches the highest per-carat prices of any opal.
White or light opal has a pale or milky body. The same colors appear softer against it, and prices sit well below black opal.
Boulder opal is precious opal still attached to the ironstone it formed in, cut with the host rock as a backing. Queensland is the source, and the dark ironstone performs much the same optical job as a black body tone.
Fire opal is the exception to the pattern: it is valued for a solid orange to red body color rather than for play-of-color, which it may show only faintly or not at all. Mexico is the principal source.
Hyalite is clear, glassy opal without play-of-color, and it earns its place on a different property entirely. Mexican hyalite from Zacatecas contains traces of uranyl compounds and fluoresces an intense green under ultraviolet light, bright enough to be unmistakable. Collectors buy it for that reaction rather than for daylight appearance. Three of the hyalites in our opal collection are Mexican and UV reactive, certified by the Czech Gemological Laboratory.
Ethiopian Welo opal arrived on the market in quantity after the Wollo Province deposits opened in 2008 and now supplies a large share of the world's precious opal. Bright, often with a strong play-of-color at prices well below Australian equivalents, it comes with one significant complication, covered in the next section.
Peruvian pink and blue opal are common opal, valued for pastel body color, with no play-of-color.
The Ethiopian Complication: Hydrophane
Most Australian opal is stable in water. Most Ethiopian opal is not, and the difference has a name: hydrophane, from the Greek for "water-loving."
Ethiopian opal is porous. Immersed in water, it absorbs it, and as it does the stone can turn transparent and lose its play-of-color entirely. Left to dry, it usually returns to normal over hours or days. Usually. Repeated wetting and drying can craze the stone permanently, and absorbed oils, soaps, perfumes, or dyes may not leave at all.
Three practical consequences follow, and they matter more than any other care advice about opal:
- Never immerse an Ethiopian opal in water, and never clean it with anything but a dry or barely damp soft cloth.
- Remove it before washing hands, swimming, or applying cosmetics.
- Treat a temporary loss of color after contact with water as an alarm, not as damage. Let it dry undisturbed rather than trying to help it along with heat.
The same porosity makes Ethiopian opal easy to treat. Smoke treatment exposes the stone to smoke from burning organic material, and carbon particles penetrate the pores and darken the body tone, which makes the play-of-color read more strongly, imitating the appearance of black opal at a fraction of the price. The treatment is stable under normal wear and entirely legitimate when disclosed. It must be disclosed, and two of the Welo opals in our collection are described as smoked for that reason. The wider framework for enhancement and disclosure is set out in our guide to treated versus untreated gemstones. Dyeing and sugar-acid treatment also occur on this material, and neither is acceptable without disclosure.
Doublets and Triplets: Read the Side of the Stone
Precious opal often forms in seams too thin to cut as a solid stone, and the trade solved that problem long ago by building composites.
A doublet glues a thin slice of precious opal to a dark backing of common opal, ironstone, or black glass, which supplies the dark body tone that a solid black opal would have naturally. A triplet adds a clear quartz or glass dome on top, which protects the fragile opal layer and magnifies the color beneath it.
Both are legitimate products sold honestly, and both are worth a fraction of a solid stone of similar appearance. The defense is to look at the stone from the side, where the glue line and the change of material are usually visible, and to ask directly whether a stone is solid. Assembled stones and the other categories that sit outside the word natural are covered in our guide to what a natural gemstone is.
Crazing, and How to Keep an Opal Alive
Opal's water content is what makes it beautiful and what makes it fragile. Lose that water too quickly and the stone develops a network of fine internal fractures called crazing, which is permanent and cannot be repaired.
Crazing is caused by dryness and by sudden temperature change, not by light. The common advice to keep opal out of sunlight is sound, but the reason is heat rather than fading; opal's color comes from structure, and structure does not bleach.
The rules that follow are short:
- No ultrasonic cleaners, no steam, no chemicals. A soft cloth and, for non-hydrophane stones, lukewarm water.
- Avoid sudden temperature changes, including a hot car dashboard and a cold windowsill.
- Store apart from harder stones. At 5.5 to 6.5 on the Mohs scale, opal is softer than the quartz in ordinary dust, so wiping a dusty opal with a dry cloth scratches it; our hardness guide explains why that number matters more than it looks.
- Set ring stones protectively, and accept that opal suits pendants and earrings better than a ring worn every day.
- Detailed handling for delicate species is covered in our gemstone care guide.
The Superstition Has an Author, and a Date
Opal is the only major gemstone whose reputation was demolished by a novel.
For most of recorded history opal was considered fortunate. Pliny the Elder praised it as combining the virtues of every other gem. Medieval Europe treated it as a stone of truth and hope. Then in 1829 Walter Scott published Anne of Geierstein, in which an enchanted opal loses its fire when holy water touches it and its wearer dies shortly afterwards. Scott never called the stone unlucky, and the plot does not require it. Readers concluded it anyway. European opal prices are reported to have fallen by around half within a few years, and the superstition outlived both the novel and its author.
The story is worth knowing for a practical reason: it is the clearest demonstration in gemology that a stone's reputation and a stone's properties are separate things, and that only one of them is a fact. Opal remains the birthstone for October, alongside tourmaline, and our birthstone guide covers the full calendar.
Where Opal Comes From
Australia dominated twentieth-century production and still supplies most fine black and boulder opal, from Lightning Ridge, Coober Pedy, and Queensland. Its share of world production, once quoted at ninety-five percent, has fallen substantially since Ethiopian material reached the market.
Ethiopia's Wollo Province, producing since 2008, is now a major source of bright precious opal, most of it hydrophane. Mexico supplies fire opal from Querétaro and hyalite from Zacatecas. Peru produces pink and blue common opal. Brazil and the United States contribute smaller quantities.
One historical source deserves a mention because it held the entire European market for centuries. The mines at Dubník in what is now eastern Slovakia were the world's principal supply of precious opal from Roman times until Australian deposits opened in the 1890s. The Harlequin Opal recovered there, weighing roughly 594 grams, remains one of the largest precious opals ever found and is held in Vienna. Every famous European opal set before 1900 is, in all probability, Slovak.
Famous Opals
The Olympic Australis, found at Coober Pedy in 1956, weighs 17,000 carats and is generally described as the largest and most valuable gem opal ever recovered.
The Aurora Australis, a 180-carat black opal from Lightning Ridge found in 1938, is regarded as the finest black opal known, with red play-of-color across a dark body.
The Virgin Rainbow, unearthed at Coober Pedy in 2003, is an opalised belemnite fossil: silica that replaced the shell of an extinct marine animal, cell by cell, and now glows across the full spectrum.
The Andamooka Opal was presented to Queen Elizabeth II on her 1954 visit to South Australia and set into a necklace and earrings.
The Flame Queen, a 263-carat black opal from Lightning Ridge found in 1918, shows a distinctive central "eye" of deep red framed by green.
Olympic Australis opal from South Australia.
Why Choose Opal
No two opals are alike, and unlike most claims of that kind, this one is structural rather than sentimental. The pattern of color in a given stone depends on how its silica spheres happened to settle across millions of years, and that arrangement is not repeatable.
Opal also asks more of its owner than almost any other gem, and rewards the attention. A stone that has to be kept out of ultrasonic cleaners, off a hot dashboard, and away from swimming pools is a stone that will be handled deliberately, and deliberate handling is why century-old opals still show their fire.
In folk tradition opal has been associated with creativity, hope, and clear sight, and it has carried those meanings in European jewelry for centuries. They belong to cultural history rather than to the physics of diffraction, and both are worth knowing.
Explore our collection of natural opals, including UV reactive Mexican hyalite and Ethiopian Welo, each listed with full treatment disclosure.
Frequently Asked Questions
What causes the play-of-color in opal?
Diffraction. Precious opal is built from microscopic silica spheres, roughly 150 to 400 nanometres across, stacked in a regular grid. White light entering the stone is split by that structure, and the wavelength that emerges depends on sphere size and viewing angle. The effect was confirmed only in 1964, when Australian researchers examined opal under an electron microscope.
Why is red the most expensive color in opal?
Because red requires the largest spheres, around 350 nanometres and above, and nature grows large spheres uniformly far less often than small ones. Blue and violet come from spheres near 150 nanometres and are common. A stone showing strong red, or the full spectrum including red, sits at the top of the market.
What is hydrophane opal?
Opal porous enough to absorb water, which describes most Ethiopian Welo material. Immersed, the stone can turn transparent and lose its play-of-color, usually returning to normal as it dries over hours or days. Repeated wetting can craze it permanently, and absorbed oils or dyes may never leave. Never immerse it, and remove it before washing hands or swimming.
What is smoked Ethiopian opal?
Opal exposed to smoke from burning organic material, so that carbon particles enter the pores and darken the body tone, making the play-of-color read more strongly. It imitates the look of black opal at a fraction of the price, is stable under normal wear, and is entirely legitimate provided it is disclosed as a treatment.
What is crazing, and can it be repaired?
Crazing is a network of fine internal fractures that develops when an opal loses water too quickly or meets a sudden temperature change. It is permanent and cannot be repaired. It is caused by dryness and heat rather than by light: opal's color comes from structure, so it does not fade.
What is an opal doublet or triplet?
A doublet glues a thin slice of precious opal to a dark backing, supplying the dark body tone a solid black opal would have naturally. A triplet adds a clear dome on top. Both are legitimate when sold as such and both are worth a fraction of a solid stone. Examine any opal from the side, where the glue line is usually visible.
Is hyalite opal valuable?
It has no play-of-color, so it is judged on other grounds. Mexican hyalite from Zacatecas contains traces of uranyl compounds and fluoresces an intense green under ultraviolet light, and collectors buy it specifically for that reaction rather than for its daylight appearance.
Is opal really unlucky?
The superstition has an author and a date. Walter Scott's 1829 novel Anne of Geierstein features an enchanted opal that loses its fire when touched by holy water, shortly before its wearer dies. Scott never called opal unlucky, but readers drew the conclusion and European prices are reported to have halved within a few years. For most of recorded history before that, opal was considered fortunate.
Can opal be worn every day?
In a ring, with caution. At 5.5 to 6.5 on the Mohs scale opal is softer than the quartz in ordinary household dust, so it scratches easily and needs a protective setting. Pendants and earrings suit it better, since they take fewer impacts and less contact with water and cosmetics.
