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Treasures from Down Under

By Benedicte and Martine Lavoie

All photos courtesy Pierres de Charme
All photos courtesy Pierres de Charme
A 4.11-carat black opal from Lightning Ridge, Australia.
A 4.11-carat black opal from Lightning Ridge, Australia.

We recently returned from a trip to Australia, the land of opals and kangaroos! There, we had the opportunity to visit several opal mines and had the pleasure of meeting with extraordinary miners, who were generous enough to show us their mines and explain the opal extraction process from A to Z.

Most mines are on private land and are rented to miners as concessions. These lands are extremely arid and mostly used for livestock. All the miners we met proved to us their work relies on patience, determination, and a certain dose of madness.

Years in the making
Opals need extremely special conditions and millions of years to form. The following is a very brief and simplified overview of these gems’ formation—a process beginning in river waters that flowed when dinosaurs still lived on Earth.

This river water, already rich in silica, would flow on sandstone beds. With gravity, the water-and-silica solution was deposited in cavities in the form of a gel, and the water slowly evaporated. The remaining silica, transformed into miniature spherules of about 150 to 700 nanometres in diameter, became trapped in the cavities. (This is why opals are often found in the form of nodules or horizontal ‘veins’ corresponding to ancient riverbeds and inland seas.)

If the silica-rich molecules form into regular spheres and ordered rows, the light diffracts, which produces a colour scheme. If they are simply deposited in no order, the result is the common opal or ‘potch,’ which forms without any colour scheme. The largest (and much less common) spherules produce red colour sets, while smaller ones produce purples and blues. Black opals showing red and orange flashes are rarer and thus tend to be more expensive.

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