How to tell if opal is real
To tell if opal is real, look side-on first: GIA shows a triplet by its seam. Gilson synthetic is opal made in a lab; plastic and glass are not opal at all.
To tell if opal is real, look at the stone side-on first, then under a lens, then weigh it. GIA shows a triplet by the seam in its profile, and the gem society says synthetic opal usually shows a mosaic pattern and does not phosphoresce. Synthetic opal is still opal: it reads 8.89 to 9.47 g in water for a 20.00 g piece.
In short
- “Real” has four meanings. A piece can be natural solid opal; assembled, natural opal glued to other materials; synthetic, laboratory-made opal; or an imitation, glass, plastic or ceramic that is not opal at all. Treated opal is a fifth case. GIA’s laboratory report states natural or laboratory-grown and names detectable treatments.
- The profile-first rule. Side-on is where assemblies show. GIA describes a triplet in profile as a thin seam of opal under a quartz dome over dyed black chalcedony. Geoscience Australia puts the triplet’s opal at about 0.5 mm, and the gem society says synthetic white opal can show columnar structures from the side.
- The lens, face-up. The International Gem Society says synthetic opal usually shows a mosaic pattern, with a cellular, scale-like, snake-skin or chicken-wire structure under high magnification; glass shows bubbles and swirl marks; black plastic impregnation concentrates colour in cracks.
- The 53-to-73 rule, worked. A 20.00 g piece of solid opal reads 11.11 g or less in water (opal). An assembly with a quartz or chalcedony cap and backing reads that low only if 53% to 73% of its volume is opal, so a triplet, its opal about 0.5 mm thick, is expected to fail the ceiling. A doublet backed with common opal passes it.
- Documented failures. Synthetic opal at the gem society’s 1.8 to 1.9 reads 8.89 to 9.47 g, inside natural porous opal’s 8.51 to 9.42 g, so weight cannot separate them. Absence of phosphorescence fits synthetics, plastic and glass, but the gem society says only that natural opal may phosphoresce.
| Step and observation | Points to | Source | 20.00 g piece in water | What it cannot exclude |
|---|---|---|---|---|
| 1. Profile: a seam, a clear dome over a thin colour layer, a black base | Triplet or doublet | GIA; GA (triplet slice about 0.5 mm, doublet slice usually over 2 mm) | Expected above 11.11 g with a quartz or chalcedony cap and base (worked below) | A closed setting that hides the profile |
| 1. Profile: columns of colour seen from the side | Synthetic white opal | IGS; Gilson columns visible to the naked eye (IGS) | 8.89 to 9.47 g at SG 1.8 to 1.9 | Synthetics that show no columns |
| 2. Lens: mosaic, chicken-wire, snake-skin or scale-like cells | Synthetic opal | IGS | 8.89 to 9.47 g | Needs high magnification and top or backlighting |
| 2. Lens: gas bubbles and swirl marks | Glass imitation, including the glass called slocum stone (GIA) | IGS; GIA | 11.30 g and up at SG 2.30 and up (IGS glass) | Which glass it is |
| 2. Lens: colour concentrated in cracks | Black plastic impregnation (treatment) | IGS: too low SG | Too low, per IGS | IGS says detection may require major laboratory equipment |
| 3. Water: above 11.11 g | Not solid opal: glass, an assembly with a quartz or chalcedony cap and backing, or another material | Opal species page ceiling; worked here | Above 11.11 g | Which of the two |
| 3. Water: 8.51 to 11.11 g | Consistent with natural, synthetic or a potch-backed doublet | IGS SG figures | Natural porous 8.51 to 9.42 g; synthetic 8.89 to 9.47 g | Synthetic against natural; doublet with an opal backing |
| 4. UV lamp: phosphorescence after the lamp is off | Consistent with natural opal | IGS: synthetics, plastic and glass do not phosphoresce | A natural opal that does not phosphoresce | |
| 5. Laboratory report | Natural or laboratory-grown; detectable treatments named | GIA | Nothing the report does not test |
How to tell if opal is real: five steps, day one to laboratory
Day one needs a 10x lens, a scale reading to 0.01 g, a thread and a cup of water. A UV lamp is step four, a laboratory step five. Each step narrows the list, and none of the first four can clear every case.
- Profile. Look at the stone side-on, out of its setting if it is loose: seams, caps and backings show here first. Below.
- Lens, face-up. Synthetic pattern, glass bubbles, colour in cracks.
- Weigh in water. The 11.11 g ceiling on the opal page, and the 53-to-73 rule for assemblies, below. Method: how to measure specific gravity at home.
- UV lamp, for phosphorescence only, as a pointer. The lamp question in general is on fluorescent minerals explained; nothing here is safety advice.
- Laboratory. GIA says its Colored Stone Identification Report identifies whether material is natural or laboratory-grown and names any detectable treatments; the gem society says plastic impregnation usually needs major laboratory equipment to detect.
The finish state is one of six labels: natural solid opal, treatment untested; assembled (doublet or triplet); synthetic opal; imitation, material named; treated, treatment named; or untested. Whether opal is a mineral at all is on is opal a mineral.
Is my opal a doublet or triplet? The profile-first rule
The profile-first rule: look at an opal side-on before you test anything else, because an assembled stone shows its layers in profile and nowhere else as clearly. GIA’s gem imitation page illustrates a triplet in profile: a thin seam of opal in the centre, backed by dyed black chalcedony and overlaid by a quartz cabochon. GIA’s opal quality page describes the doublet as a thin layer of opal cemented to a backing of obsidian, dyed black chalcedony, black glass, natural common opal or plastic, and the triplet as a thin layer of opal between a domed top of colourless quartz or clear glass and such a backing. The imitation page adds rock crystal, plastic, glass or synthetic corundum as dome materials.
How thin. Geoscience Australia says triplets use slivers of precious opal about 0.5 mm thick, and that a doublet’s slice is usually greater than 2 mm. GIA notes that assembled opals are not always imitations: they are partly precious opal, made from pieces too thin to use alone. An assembled stone is real opal plus something else; the label should say so.
Where the rule fails. A bezel or closed setting hides the profile, and a doublet backed with natural common opal is opal on opal, so the join may be the only sign. The reverse trap is natural: GIA says boulder opal is cut with its host-rock matrix attached, so a solid natural stone can show opal over rock in profile. A layered profile means look harder, not assembled. The general imitation sequence is on how to tell if a crystal is fake.
Is synthetic opal real opal? Gilson and the lens
Synthetic opal is opal made in a laboratory, and the sources differ slightly on how identical it is. The International Gem Society says synthetic opal has the same chemical and physical properties as mined opal, so it is real opal, and that Pierre Gilson began to market it in 1974, with columns of colour easily visible even to the naked eye. GIA’s synthetics page says the Gilson Company developed a three-step process in the 1970s, precipitating microscopic silica spheres, settling them in acidic water for more than a year and consolidating them in a hydrostatic press, and that in synthetic opal, as in synthetic turquoise, additional compounds can be present.
What the lens shows. The gem society says synthetic opal may show a strong display of colour, usually in a mosaic pattern; with high magnification and top or backlighting, a cellular, scale-like, snake-skin or chicken-wire structure; under high magnification with transmitted light, sometimes a dendritic structure; and, in synthetic white opal, columnar structures from the side. GIA says Gilson opals may look like highly valuable natural white and black opal to the unpractised eye.
The documented failure: the scale. The gem society gives synthetic opal a specific gravity of 1.8 to 1.9 and natural porous opal, often from Ethiopia, 1.74 to 1.89. Worked as 20 − 20/SG, a 20.00 g piece reads 8.89 to 9.47 g as synthetic and 8.51 to 9.42 g as porous natural opal. The ranges overlap, so weighing cannot separate them; the lens and the laboratory can.
Does weighing catch a triplet? The 53-to-73 rule, worked
The 53-to-73 rule: an assembled opal with a cap and backing of quartz or chalcedony weighs in under the solid-opal ceiling only if 53% to 73% of its volume is opal. The opal page sets the ceiling: a 20.00 g piece of solid opal reads at most 11.11 g in water, the USGS top figure of 2.25. An assembly’s specific gravity is the volume-weighted mean of its parts, so the opal fraction f needed to reach 2.25 is (SG of the other parts − 2.25) ÷ (SG of the other parts − SG of the opal), with the glue ignored.
Worked. With quartz at 2.65 and opal at the gem society’s 2.10 for black and white opal, f is 73%; with opal at 1.98, the USGS low figure, 60%. With chalcedony at the gem society’s low figure of 2.55, f is 67% or 53%. For a triplet’s opal, a sliver about 0.5 mm thick on Geoscience Australia’s figure, to reach 53% of the volume, the dome and backing together would have to take up less room than the sliver. So on these inputs a triplet is expected to read above 11.11 g: the ceiling flags it as not solid opal, correctly, although it contains opal. Substitute your own layer thicknesses; the formula is the method.
Where it fails. A doublet backed with natural common opal is all opal and weighs as opal, 10.48 g at SG 2.10. With a black glass backing at the gem society’s lowest glass figure, 2.30, as little as 16% to 25% opal brings the stone under the ceiling. GIA also lists plastic backings and domes, and no source fetched for this page gives plastic a density, so the rule says nothing about them.
Glass, plastic, slocum stone and treated opal
Imitations are not opal at all. The International Gem Society opal article says imitations are typically plastic or glass; plastic ones are soft and can be probed with a sharp needle, which marks the stone and is not a test we suggest; glass ones contain bubbles and swirl marks and usually have higher specific gravity and refractive index than natural opal. Neither phosphoresces. GIA says a glass material known as slocum stone can mimic opal. The gem society’s simulant article lists plastic and glass opalite, plastic resin, slocum stone and a Kyocera imitation among its headings; its full text is for members and was not read. The gem society’s opal article says the word opalite usually refers to a plastic or glass simulant.
Treated opal is opal. GIA’s opal buyer’s guide says opals can be treated by impregnation with oil, wax or plastic and by surface modifications. The gem society lists oil, wax or plastic impregnation; black plastic impregnation, detectable by too low an SG and colour concentrated in cracks; smoke impregnation, with low SG, an index of 1.38 to 1.39, and play-of-colour that disappears when wet; and aniline dye, silver nitrate or sugar treatments, showing black concentrations under magnification.
What this page leaves out. The gem society mentions a hot-point test for oil and wax and says synthetic opal may stick to the tongue; neither is given here as a method. No care, storage or soaking advice is given. This page gives no buying advice, and the other identification guides are at the collecting guides. Nothing on this site is offered for sale; the wanted list is the only commercial route.