01 August 2018

We have just returned from an absolutely fabulous trip to the land of opals and kangaroos! We had the opportunity to visit several opal mines as well as one of the largest sapphire mines in Australia, which will be featured in our next newsletter. We had the pleasure to meet and discuss with extraordinary miners who were so generous in showing us their mines and explaining 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 that their work relies on patience, determination and a certain dose of madness!

The formation of opals

Opals need extremely special conditions and millions of years to form. Here’s a very brief and simplified overview of the formation of opals; a process that began in river waters that flowed at the time when dinosaurs still lived on Earth.

River waters, already rich in silica, flow on sandstone beds. With gravity, the water and silica solution deposit in cavities in the form of a gel, and then slowly water evaporates. The remaining silica, transformed into miniature spherules (of about 150 to 700 nanometers in diameter), gets trapped into 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 molecules form into regular spheres and ordered rows, then the light diffracts, which produces the colour play. If they are simply deposited in no order, it is the common opal or "potch" that forms without any colour play. The largest (and much less common) spherules produce red colour while smaller spherules produce purples and blues.

An overview of the different types of opals

The world of opals is absolutely fabulous. A wide variety of opals can be found all over the world but it is those of Australia that are the most well known and most often used in jewellery.

White opals

White opals are the most common precious opals (with colour play). They are found southwest of Brisbane. The base of the opal ranges from white to very light gray.

Black opals (and "dark opals")

Black opals are found exclusively at Lightning Ridge in New South Wales, Australia. This is where most mines are located, but black opals can be found all along the ridge, which is about 100 km long. The colour play of the black opal is particularly noticeable because of its dark background and the contrast with colour flashes. These are the most expensive opals in the world but also the most rare!

Boulder opals

Boulder opals are found in the ironstone. The precious material is often found in veins or layers that are too thin to be extracted from the rock. Unlike black and white opals, which are entirely made of opal, Boulder opals are carved into the ironstone. These are often larger and less expensive than black opals, although their colour play is absolutely dramatic with their relatively dark background.

Matrix opals

Matrix opals are like small veins in the ironstone. Their formation is pretty much the same as Boulder opals’ but with veins that are thinner (which are actually fissures filled with silica that have become opals).

Opalized fossils

Since the opal forms inside porous material, it can be found in bone or shell fossils or in fossilized wood.

Lightning Ridge

Lightning Ridge is a town located 743 km southwest of Brisbane. Its economy is mainly based on mining, opal sales and tourism. Indeed, many Australian tourists visit this region during winter. It is there that Jack Murray found the first black opals in the year 1900.

Lightning Ridge opals are found about 6 to 18 meters deep, often very close to the contact zone between the sandstone that’s on the surface and the clay at the bottom. Each miner is entitled to two concessions of 50 meters by 50 meters that they can renew each year.

Once the miner has found a location he has 28 days to study the ground before leasing the concession. He analyses the soil by digging samples that are up to 10 meters deep. Once he has his concession, he then digs a hole about 20 to 30 meters deep by about 1.5 meters in diameter.

The miner carries down his equipment, as the hole gets bigger. In order to solidify the mine he installs vertical pine trunks.

The crushed rocks are vacuumed up to the surface through a large tube and then loaded directly into a truck. At the end of the day, the miner unloads his truck in what is called an agitator, old mixer truck or cement mixer. The rocks rotate slowly for 2-5 days in water that dissolves clay and sandstone. When the process is finished, only common and precious opals remain.

Yowah 

A small town of 50 permanent residents located about 1000 km from Brisbane, Yowah is where we find the "Yowah Nuts".

These opals are found in ironstone “nuts” the size of a small avocado. It is when the “nuts” are sawed in two that opals are found, either in their center or in the form of veins. Their formation was a mystery up until geologists discovered that these rocks once used to be more porous, thus allowing silica-laden water to enter.

We had the pleasure to meet Eddie Maguire who had the generosity to show us a great part of his collection of opals. This artist has been working opals for many years. Most of the time they are simply sawed in half to show their exceptional beauty and placed on a custom carved wooden base.

Like Boulder opals, Yowah opals are rarely completely extracted from the source rock. It is also in these "nuts" that we find Matrix opals.

Quilpie

Probably one of the most significant parts of our trip was visiting Eric Stelzer’s mine, which is about 2 hours from Quilpie and accessible only by a bumpy dirt road.

We had the chance to stay at the mine for two days. This was the first open sky opal pit that we visited, as most of them lay underground. It is there that Boulder opals are found. Miners know how deep the opals are, roughly close to old riverbeds.

Just like in Lightning Ridge and Yowah, the soil there is mainly sandstone and clay; we can distinguish ironstone boulders trapped in the ground. The miner breaks a few on the spot to see their potential before bringing larger amounts to the camp. Often he can already notice some veins of colours.

On the second day of our stay at the mine, Eric took advantage of the presence of his son Colin, who is a geologist, to probe another mine about 45 minutes away. With the help of a drone they were able to take pictures of the topography of the place.