How Opal Formed in Australia

About 100 million years ago, during the Cretaceous, a shallow inland sea called the Eromanga Sea covered much of central Australia. It was cold, muddy and starved of oxygen — bad conditions for most things, but exactly right for opal. Diatoms living in the water carried silica in their cell walls, and as the sea slowly retreated it left behind vast beds of silica-rich sediment. Groundwater carried that dissolved silica down into cracks and cavities in the rock beneath, where it settled into a gel and began to harden — a process so slow that it takes roughly five million years to form a single centimetre of opal. That one retreating sea, and the handful of fields left in its wake, is why Australia still produces around 90% of the world's opal today, and why a stone from Coober Pedy never quite looks like a stone from Lightning Ridge.

Coober Pedy

Opal was found here in February 1915 — the story goes that fourteen-year-old Willie Hutchison turned up the first piece while his father, Jim, was out prospecting for gold nearby. What grew up around that find isn't really one field but dozens, each staked and named separately — Seventeen Mile, Five Mile, Dead Horse Gully, Shell Patch, and more spread across the surrounding desert, some of them still worked today. Between them, Coober Pedy is the largest opal mining town in the world, responsible for something like 70% of the planet's white and crystal opal. Its name comes from the local phrase kupa piti, usually translated as "white man's hole" — a fair description of a town where a large part of the population lives underground. It wasn't a stylistic choice: summer temperatures here regularly clear 40°C, and returning WWI soldiers, already used to dugout life in the trenches, found that cutting a home straight into the sandstone kept it a steady 22–24°C no matter what the surface was doing. A second wave of miners arrived from Europe after WWII. Our white and light opal is attributed to this field.

Lightning Ridge

Black opal — the rarest kind there is — had been noticed here since the 1870s, but it took until the turn of the century for the field to properly open up. Jack Murray, a boundary rider, became the first registered miner in 1901. The following year a prospector named Charlie Nettleton walked 700 kilometres from White Cliffs just to see the new find for himself; in 1903 he pulled a large parcel of quality opal out of an underground working, then walked the same 700 kilometres back to build a market for it. That round trip is more or less how the town got made. The ground here is worked in named claims anywhere from one to forty hectares, some mined underground to depths of twenty-five metres, others by open cut. Miners are still called gougers, a word that sums up the field's whole economy in one syllable — some pull out a fortune in a few months, others come away with nothing for just as long. What makes Lightning Ridge opal so prized is its body tone: trace carbon and iron oxide locked into the silica as it formed, dark enough that the colour trapped inside reads as a flash of neon against black rather than a softer shimmer against white. We attribute each black opal to the Ridge specifically.

Mintabie

Mintabie's opal ground was first worked in 1935, on what old-timers still call the Old Field. It sits on Anangu Pitjantjatjara Yankunytjatjara Lands in the far north of South Australia, leased to the state government from the early 1980s under the Pitjantjatjara Land Rights Act, and a run of strong finds sparked a second, much bigger opal fever there in 1978. The field was worked for another four decades after that. In 2019 the lease ended and the land was handed back to its traditional owners — mining there stopped for good, and the town itself was decommissioned. What we hold is existing stock, cut before the field closed. There will be no more of it, ever, which is a strange thing to be able to say about a gemstone with any certainty.