Hold a finished opal up to the light and it is easy to forget the stone took millions of years and a long run of luck to come together. People ask me how opals are made far more than they ask about any other gem, and it is a fair question, because opal forms in a way no other precious stone does. Here is the short version of how nature builds one, and how it is dug out of the ground.
How opal forms
Opal begins as silica dissolved in water. Rain works its way down through sandstone and other rock, picking up silica as it goes, and carries it into cracks, cavities and pockets underground. As the water slowly evaporates or the conditions change, the silica is left behind as a gel that hardens over time. Where the tiny silica spheres settle into a tidy, even arrangement, you get precious opal with play-of-color. Where they pile up at random, you get common opal with none. That single difference, order versus chaos at a microscopic scale, separates a museum stone from a paperweight.
How long it takes
A long time. The general estimate is that it takes nature something like five to six million years to make a band of opal just a centimeter or so thick. The conditions have to stay right across that whole stretch, which is part of why fine opal is scarce. It is not a stone you can hurry, and despite a lot of effort no one grows convincing precious opal in a lab at any scale.
Where it forms
Most of the world’s opal is sedimentary. It formed in the ancient inland sea beds of Australia, where the Great Artesian Basin gave the water and the rock exactly the right setting over millions of years. That is why Australia has led the trade for so long. A smaller share is volcanic in origin, formed in cavities in lava. Mexican fire opal and much Ethiopian opal come from volcanic settings, which is one reason they behave a little differently from Australian material. I compare the two in Ethiopian vs Australian opal.
Want a stone that took millions of years to make? See the handcrafted opal rings in my collection, or talk to me about a custom piece.
How opal is mined
Opal mining is still a small-scale, hopeful business, closer to prospecting than to industry. At Coober Pedy in South Australia, the home of white and light opal, miners sink shafts and tunnel through the levels where opal tends to sit, then sort through the rock by hand. The town is so hot that many people live underground. At Lightning Ridge in New South Wales, the source of the finest black opal, it is much the same: shafts, tunnels, and a great deal of patience. Spoil heaps are picked over again for missed color, a practice the miners call noodling.
Boulder opal and opalized fossils
In Queensland the silica seeped into ironstone boulders rather than open ground, and the result is boulder opal, cut with its natural host rock left on the back. Sometimes the silica filled a space left by something that had once been alive, a shell or a bone or a piece of wood, and replaced it color for shape. Those are opalized fossils, and they are among the most remarkable things the ground gives up.
Why every opal is different
Because opal forms by filling whatever space happens to be there, no two stones are ever truly alike. The pattern, the colors, the shape of the rough, all of it is set by the accidents of where the silica went and how it settled. That is the part I have never tired of in fifty years. Every parcel is a surprise.
Frequently asked questions
How long does it take for opal to form? Roughly five to six million years for a centimeter of opal, under the right conditions held steady over that whole period.
Where do most opals come from? Australia produces the great majority of the world’s precious opal, chiefly from Coober Pedy, Lightning Ridge and the Queensland boulder fields. Ethiopia and Mexico are the next best known sources.
Can opal be made in a lab? Synthetic and imitation opal exists, but genuine precious opal is not grown commercially at any real scale. Almost all opal in the trade is natural.
For the bigger picture of what opal is and how to choose one, see my guide to understanding opal. Or browse the current opal rings, each set with a natural stone I chose by hand.
