Opals are discovered in any part of the world, but just a handful of places are considered significant producers of the gem. Most of the world’s opal comes from Australian opal, where production is around 95%. This sedimentary rock has flourished in Australia, where there are certain particular geological features.

Opal may be found in wide different varieties in deposits all over the world. Fire-like reds, oranges, and yellows are characteristic of opal. Boulder Opals consist of opals that have been bonded to host rock. It’s more attractive than you may think! Even though its name suggests otherwise, the colour, pattern, and brilliance of a common opal may make it anything from ordinary. Who decided on such a generic label?

Regarding mineral classification, opal is a kind of hydrous silicon dioxide with the formula SiO2.nH2O. It lacks a crystalline structure and a defined chemical makeup, making it amorphous (it contains a variable amount of water, as shown by the “n” in its chemical composition). Opal is not a true mineral but rather a “mineraloid” for this reason.

Properties 

Opals are multi-coloured gemstones made up of tiny silica spheres arranged in a pattern with gaps between them that are filled with water. When white light travels through the spheres, it is broken into component colours and becomes more diffuse. The term opalescence is used to characterise this transition. Larger spheres are responsible for producing all colours, but smaller spheres can only create blues and greens. Red opals are now only found in highly unique settings due to the increased production of silica spheres.

White opals are characterised by soft, pastel tones set on a white backdrop. Black opals are highly uncommon and costly because of their black background and vivid colours ranging from red to green to blue to purple. Boulder opals have a back made from the original ironstone host rock.

That Huge Basin of Artesian Waters

About 140 million years ago, the formation process began. At that time, much of central Australia was covered by an inland sea full of silica-rich sands. The Great Artesian Basin is the remnant of this once-vast sea.

An intense weathering began dissolving the silica 30-40 million years ago. Water rich in silica started collecting in the holes drilled into the rock. Opals were produced throughout time from silicates that were trapped there.

The Great Artesian Basin is home to very few opal-producing locations despite its size. Any miner will tell you that high-quality opal is challenging to find, even in the most promising regions.

Several prominent opal fields in Australia, inside and outside the Great Artesian Basin, are shown on the map to the right.

Origins of the Australian Opal Industry

Modern opal mining in Australia began in July 1889. Tullie Cornthwaite Wollaston transported sixty pieces of dazzling opal rough from a Queensland mine to the British capital.

After Wollaston sold the stones to a global jewellery company, demand for Australian opal gemstones increased gradually worldwide, despite early rejection from gem dealers.

Opals from Preferred Mines

• Lightning Ridge, New South Wales and Mintabie, South Australia, are the most common sources of black opal (stones with a dark grey to blue-black background). Black opal is also mined in Coober Pedy, South Australia.
• White opal and other gems may also be abundant at Coober Pedy and Mintabie (stones with a white or light background).
• Several places in Queensland have boulder opal, a rich opal seam on an ironstone substrate. Similarly, Andamooka, South Australia, has produced several more minor discoveries.
• ‘Yowah’, Queensland, is home to “Yowah nuts,” which are opal-filled stones the size of walnuts.