Quick answer
Opal is a hydrated form of silica, it actually contains water, and it is not a true crystal. Precious opal flashes with rainbow colour ("play of colour"), while common opal does not. It is fairly soft (5.5-6.5 on the Mohs scale) and water-sensitive, so it needs gentler care than quartz. Watch for doublets, triplets and man-made "opalite" glass, which are not the same as a solid natural opal.
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Few stones are as captivating, or as misunderstood, as opal. It can flash with every colour of the rainbow, yet it is soft, full of water, and not technically a crystal at all. That mix of beauty and delicacy is exactly why it pays to know what you are buying. This guide explains what opal really is, what creates its famous play of colour, the main types and colours, how to tell a solid natural opal from a doublet, triplet or man-made imitation, and how to care for it.
What is opal?
Opal is a hydrated form of silica, with the formula SiO2·nH2O, which simply means silica plus water. That water content is unusually high for a gemstone: typically between 3% and 21%, and often around 6-10%. Unlike quartz, opal has no ordered crystal structure, so strictly speaking it is a mineraloid rather than a crystal. It is still a completely genuine natural stone, just an amorphous one, formed when silica-rich water seeps into cracks and cavities and slowly sets into a solid gel.
This structure has two big consequences. First, it is what makes precious opal flash with colour. Second, it makes opal relatively soft and delicate: it sits at just 5.5-6.5 on the Mohs scale, well below quartz, and because it holds water it can crack or "craze" if it dries out or is exposed to sudden heat. The name comes from the Latin opalus, likely from the Sanskrit upala, meaning "precious stone."
What causes the play of colour?
The rainbow flashes in precious opal are not a pigment, they are an optical effect. Inside the stone are millions of microscopic silica spheres stacked in an orderly grid. When light passes through that grid it is diffracted, split into its component colours, just like a thin film of oil on water. The size of the spheres decides which colours you see: smaller spheres flash blues and violets, larger ones bring in greens, oranges and reds (red play of colour is the rarest and most prized). Move the stone and the flashes shift, because the angle of the light changes. Common opal lacks this orderly structure, which is why it shows a solid body colour but no play of colour.
Types and colours of opal
Opal splits into two big groups: precious opal, which shows play of colour, and common opal (also called "potch"), which shows a solid colour but no flash. Within those groups you will meet:
- White (light) opal, a pale milky body with play of colour; the most familiar type.
- Black opal, a dark body tone that makes the colour flashes pop; the most valued precious opal.
- Boulder opal, thin seams of opal still attached to their natural ironstone host rock.
- Crystal opal, transparent to translucent with play of colour shining through.
- Fire opal, a warm orange to red body colour (it may or may not flash), famous from Mexico.
- Common opal, including pink, blue and green opal, prized for their soft, solid colours rather than flash.
Because each piece forms differently, no two opals are alike. Browse our opal collection to see the range.
Real, assembled or imitation: what the words mean
With opal it really helps to separate three things, because they are not the same:
- Solid opal, a single piece of natural stone. This is what most people mean by "real opal."
- Doublets and triplets, assembled stones: a thin slice of genuine opal glued to a dark backing (doublet), sometimes with a clear quartz or glass cap on top (triplet). The opal is real, but the piece is built up, which keeps it affordable. A good seller always discloses this.
- Synthetic opal, grown in a lab (such as Gilson opal). It has real opal's structure but is man-made, and usually shows a very regular, "too perfect" colour pattern.
- Imitation "opalite", which despite the name is not opal at all, it is manufactured glass or resin. The same goes for "sea opal" or "galaxy opal" resin pieces.
None of these are bad to own as long as they are described honestly. The only real problem is paying a solid-opal price for a triplet, a synthetic, or a piece of opalite glass. For more on this, see our guide on how to tell if your crystals are real.
Is opal durable? Care and water
Opal needs more care than most stones, for two reasons: it is soft (5.5-6.5 on the Mohs scale, so it scratches and chips more easily than quartz, which you can compare in our guide to the most durable gemstones), and it contains water, which can be lost. If an opal dries out or meets sudden heat or a big temperature change, it can develop fine internal cracks called crazing.
To keep opal safe: keep it away from heat, direct sunlight for long periods, and very dry air; never use ultrasonic or steam cleaners; avoid harsh chemicals, perfume and hairspray; and clean it only with a soft, slightly damp cloth. Solid opal can take a brief, gentle rinse, but doublets and triplets must not be soaked, because water can seep between the layers and cloud them. A special note for Ethiopian Welo opal: it is "hydrophane," meaning it absorbs water and can temporarily go clear or lose its flash when wet, returning as it dries, so keep it dry. Because opal is delicate, it is happiest in pendants and earrings; in rings, choose a protected setting and treat it gently. See our water-safe crystals guide and how to store crystals.
How to choose opal
Opal is judged on its colour and character rather than clarity. A few things to look for:
- Play of colour (for precious opal), broad, bright flashes that shift as you move the stone; reds and a wide colour range are the most prized.
- Body tone you like, from milky white to dark "black," or a warm fire-opal orange.
- Honest disclosure of whether the piece is solid, a doublet, a triplet, synthetic or imitation.
- A protective setting if you plan to wear it daily, since opal is soft.
- The occasion, opal is the birthstone for October, which makes it a popular and thoughtful birthday gift.
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Frequently asked questions
Is opal a crystal?
Not technically. Opal is amorphous (a mineraloid) with no crystal structure, but it is still a genuine natural stone made of hydrated silica.
Why does opal flash with colour?
Tiny silica spheres inside precious opal diffract light into its component colours. Common opal lacks this orderly structure, so it shows colour but no flash.
Can opal go in water?
Solid natural opal can take a brief, gentle rinse, but never soak doublets or triplets, and keep hydrophane (Ethiopian) opal dry, as it absorbs water and can lose its flash until it dries.
Is "opalite" real opal?
No. Opalite is man-made glass or resin, not opal. It is fine to own when sold honestly, but it should never be priced as natural opal.
Is opal too soft to wear?
It is softer than quartz (5.5-6.5 Mohs), so it suits pendants and earrings best. In rings, choose a protective setting and avoid knocks and chemicals.
