Recognising Real Opal: Natural, Doublet, Triplet or Synthetic
Precious opal, common opal, doublet, triplet, laboratory opal or "opalite" glass: the pointers that tell you what you are buying, without risky tests.
Key points
| Nature | hydrated silica (SiO2 + water), non-crystalline: a mineraloid |
|---|---|
| Hardness (Mohs) | about 5.5 to 6.5 |
| Water content | 3 to 21%, most often 6 to 10 % |
| Density | about 2.0 to 2.2 |
| Play-of-colour | diffraction of light by silica spheres 150 to 300 nm in diameter |
| Cases to distinguish | precious opal, common opal, doublet, triplet, synthetic, glass |
| Sources | Australia (the majority of production), Ethiopia, Mexico, Brazil, United States, Slovakia |
| Precaution | sensitive to heat, knocks and dehydration; do not soak (hydrophane opal) |
Opal: a fragile stone with many faces
Opal is silica containing water, with no crystalline structure in the strict sense: it is classed as a mineraloid. Precious opals show play-of-colour: flashes of red, green and blue that change with the angle. They come from tiny silica spheres, regularly stacked, which diffract light. This mechanism was demonstrated under the electron microscope by J. V. Sanders in the mid-1960s. Common opal shows no play-of-colour: it is simply milky, orange (fire opal), pink or bluish.
Australia supplies the majority of the world's precious opal (95% is the figure often quoted, probably an overestimate). Ethiopia (Wollo province) has produced it in large quantities since 2008, as have Mexico (fire opal), Brazil, Nevada and Slovakia.
Because opal contains water, it is sensitive: very dry air can make it crack (known as "crazing"), and sudden heat or knocks can damage it. Its hardness is 5.5 to 6.5.
What you are sold as opal
| Product | What it is | Honest if... |
|---|---|---|
| Natural precious opal | Opal with play-of-colour, cut from a single piece | Its nature is stated |
| Doublet | A thin slice of opal glued to a dark backing (potch, ironstone, onyx, obsidian) | "Doublet" is written on the product page |
| Triplet | A doublet capped with a dome of clear quartz or glass | "Triplet" is written |
| Darkened by treatment | Pale opal blackened by smoking (Ethiopian opals) or by a sugar treatment on the rock | The treatment is disclosed |
| Impregnated opal | Porous opal stabilised with resin or plastic | The impregnation is disclosed |
| Synthetic opal | Made in a laboratory (Pierre Gilson process, from 1974) | "Synthetic" is written |
| "Opalite" glass | Milky bluish glass, unrelated to opal | It is sold as glass |
There are also glasses with artificial play-of-colour, sold as "Slocum stone" or under other names: these are manufactured products, not opals.
Warning signs
- A glue line or a dark backing visible from the side: doublet or triplet. Look at the piece from the side, not only from the front.
- A heavily domed glass cap over a flat stone beneath: triplet.
- A "snakeskin" or "chicken-wire" pattern under the magnifier, with regular coloured patches and sharp edges: synthetic opal (this pattern is rare or absent in natural opal).
- Colours that are too perfect, with no real play-of-colour: a uniform bluish sheen on a milky background suggests glass opalite.
- A weight that is too low or too high for its size, or a warm feel: plastic or glass.
- A very low price for a black opal or one with strong play-of-colour: fine pieces are expensive.
Simple, safe tests
- Read the description: "natural opal", "doublet", "triplet", "synthetic", "opalite".
- Look from the side under the magnifier: look for the dividing line and the dark backing.
- Turn the piece under a lamp: genuine play-of-colour changes hue in irregular patches as the angle changes; a reflection that never varies is suspicious.
- Observe the surface pattern under the magnifier (snakeskin, chicken-wire) or round bubbles (glass).
- Do not wet it as a rule. Some opals, known as hydrophane (notably from Ethiopia), absorb water: they become translucent or change colour, then turn milky again as they dry. A water test serves no purpose and can also damage them. No scratch test either.
Avoid heat (lighter, oven), chemical products, ultrasound and steam.
What these tests do not prove
The tests above do not always distinguish a natural opal from an impregnated opal or a well-made synthetic. Laboratories use microscopy, luminescence and spectroscopy, among other methods. For a valuable piece, ask for a certificate from a recognised gemmological laboratory.
A doublet or triplet opal is not "fake": it is an assembled piece, honest if it is disclosed, and often more affordable. See our comparison of natural, doublet and synthetic opal and our opal page for care. Our product pages state the nature of each piece: see our commitments.
Frequently asked questions
How can I recognise real opal?
Look at the play-of-colour in the light: it changes in patches as the angle changes. View the piece from the side to spot a doublet or triplet, and under the magnifier to spot a laboratory pattern or bubbles. For a valuable piece, a certificate is needed.
Is opalite a form of opal?
Usually not: opalite is milky bluish glass, sometimes also used to describe common opal. Ask what the material is before you buy.
What is an opal doublet?
A thin slice of opal glued to a dark backing. The play-of-colour is real, but the piece is assembled. It must be sold as a doublet.
Can I wet an opal?
It is better to avoid soaking. Some opals absorb water (hydrophane opals) and may change appearance, then crack. A soft cloth, slightly damp, is enough for cleaning.
Can an opal crack?
Yes. Because it contains water, it can crack if the air is very dry or if heat is sudden. Store it away from rapid changes in temperature.
Also read
The meanings mentioned belong to tradition and symbolism. They are neither promises of results nor medical advice. Back to Recognising real stones.