Understanding Boulder Opal

Deep in the Australian outback, miners tirelessly labor through tons of rock in the hope of finding a fine source of boulder opal. But what exactly is boulder opal, and why is it so desirable? In this article, we will explore the highly-prized desert gem, providing insights into its origin, formation, mining process, and value.

What is Boulder Opal?

Boulder opal is a spectacularly colorful form of opal primarily found in Queensland, Australia. It is also found in other locations such as New South Wales, Canada, and Brazil. This type of opal is known to be harder and more stable than most precious opal, making it highly desirable.

Different colors of boulder opal, such as blue and green, are popular, but red is always the most valuable and rarest color. The colors may change when the viewing angle is changed. On the Mohs scale, all opals have a hardness of 6.

What is so Special about Boulder Opal?

Boulder opal from Queensland is typically found in areas around Winton, Quilpie, and Yowah. It is usually found in fractures and spaces within impermeable sandstone or ironstone. The natural ironstone base of boulder opal adds stability and depth of color to the gems. The gems also occur in varied and interesting natural shapes, making them ideal for unique and captivating jewelry.

A 46 carat gem boulder opal specimen. Image credit: Cody Opal

Different areas of Queensland and layers within the same formation may produce slightly different types of opal, depending on specific conditions during their formation. For example, Yowah produces opal that looks like a nut, while “fairy” opal is a small smattering of opal in sandstone that gives the impression of “fairy dust” when wet. These effects are achieved through treatments with sugar, acid, and varnish to darken the stone and enhance the colors.

How Does Boulder Opal Form?

Boulder opal is formed in cracks, crevices, and cavities. Silica from dissolved sand trickles down cracks and crevices in ironstone boulders, and the silica precipitates out of solution. The opal is typically found within boulders that need to be broken open, located in clay layers just above the ironstone.

An example of boulder opal in its host rock. Image by Barbara Kolator FGA DGA.

There are various theories on how boulder opal forms, with one suggesting the presence of pyrite. When pyrite is dissolved by rainwater, it produces sulphuric acid that dissolves silica in quartz, reacts with feldspar, and releases more silica. Colloidal silica then fills fractures in ironstone and solidifies to form opal. Other theories involve bacteria, groundwater, sand, and specific minerals.

How is Boulder Opal Mined?

Mining for boulder opal in Queensland typically involves individuals on relatively small claims using an “open cut” process. Earth-moving machinery is used to remove the overburden, exposing the opal layer and opal-bearing boulders. These boulders are then cracked open to see if they contain any opal.

The opal level is a layer of white clay-like material just below the sandstone. Different layers and deposits can yield different types of opal. The opal is cut into slabs using a diamond saw, trimmed to expose the opal area, and ground down to reveal the face. It is then sanded using progressively finer grits until a polished surface appears. This process adds value to the opal before it is sold.

A digger used for opal mining in Queensland, Australia. Image by Barbara Kolator FGA DGA.

Miners are responsible for all aspects of the mining process, from moving earth and breaking boulders to sorting, cutting, and polishing the opal. Cutting is often done in a caravan at the mine site using solar power. Many miners also create jewelry from their own stones.

At the end of a mine’s life or when the lease needs to be returned, everything must be put back as it was. The land is refilled, and any dwellings are removed. Due to the natural vegetation growth in the region, it becomes difficult to tell that a mine once occupied the site.

The Value of Boulder Opal

Determining the value of boulder opal can be challenging, but generally, the piece becomes more expensive as it exhibits more colors. A black opal specimen is more valuable than a light-colored one, and the order of color value is red, orange, green, blue, and purple.

Boulder Opal Care and Caution

Boulder opal does not require any special care as it is quite stable and safe in water. However, it is important to handle it with caution to avoid dropping it onto a hard surface or chipping it. If worn in rings, it is recommended to remove them before engaging in activities such as cleaning, gardening, or manual work.

Blue opal doublet. Image by Pat Daly FGA DGA, Gem-A.

To differentiate between boulder opal and doublets, it is necessary to examine the side of the stone to see if the edge is undulating. Some doublets and triplets use an ironstone or sandstone base, making them hard to identify once set in jewelry.

In conclusion, boulder opal is a highly-prized gem found in the Australian outback. Its unique colors, stability, and natural formations make it a desirable choice for jewelry enthusiasts. Whether you appreciate the vibrant blue and green hues or seek the rarity of red, boulder opal offers a captivating and valuable addition to any collection. Take care of your boulder opal to ensure its long-lasting beauty and enjoyment.

Cover image: A fine example of recently mined boulder opal. Image by Barbara Kolator FGA DGA.

Find out more about opal on Gem-A’s Gem Hub. Expand your gemmology knowledge by signing up for our Gemmology Foundation course and become a diamond expert with Gem-A’s Diamond Diploma course.

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