Koroit Boulder Opal

It has all the precious opal flashes, contained in a rusty matrix.

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Story and Photos by Russ Kaniuth

When you say “opal”, most people think of a continuous stone sparkling with color flashes from edge to edge. Australian Koroit boulder opals have all the precious opal flashes, but they’re contained in a rusty matrix with many variations and patterns. I spoke to Comet mine owner Gene McDevitt not too long ago, and he gave me more info about the Koroit opal field and the material itself.

Koroit opals are found in a unique geological area in South West Queensland, Australia. The mining area itself is only approximately 10 miles square and seems to contain nothing but dirt and poisonous snakes. Underground, however, is something entirely different. The opals can be anywhere, but are usually found anywhere from the surface to 70 feet underground, where the sandstone and clay interface. The opals are found within concretions at this layer.

The matrix generally consists of ironstone, a type of sandstone that is rich in iron, but also contains other minerals such as manganese. The patterns vary so much that no two concretions are alike. The pattern that most lapidaries look for is called “the grassy pattern”. It almost has a Celtic design to it, filled with precious opal with brilliant colors. Other patterns will have a swirly mixture of colorful lines that contrasts nicely with the dark matrix stone.

Before cutting your first high-grade Koroit boulder opal, it’s highly recommended that you experiment with lower-grade material first. This material is unlike any other and does take a little time to get used to. After cutting a few batches and getting used to how to cab this material without wiping out all the opal veins, then it would be time to step up to a higher grade.

The first thing to do before cutting is to examine the stone carefully, both dry and wet. Look for streaks running through the stone; these are generally the opal veins. I start by cutting 3 mm to 4 mm below that line and about 2 mm to 3 mm above it. This way, I will have a flat back. Then, I start to grind down the top until I start reaching the colorful opal layer.

Since the mixture of matrix can have such drastic differences in hardness and the opal is in thin layers, I would suggest never using a coarse-grit wheel on this material. Start out on the 220 grit to achieve your shape. Many times, you won’t have much choice in what shape you’d like it to be; the stones are generally small, and there’s always the chance that pieces will crumble off until you get down to the solid portions that can be cabbed.

Here is where you start to experiment, by grinding down until you hit color and seeing how far down you can go in order to shape and dome the cab without grinding away all the opal. Remember, most of the opal layer is very thin, so it takes a delicate hand. Continue to check to make sure you don’t grind away all the color.

Once you have achieved a desired shape and dome, continue your path through the higher-grit wheels, but go slowly and gently, as even the lighter grit wheels will remove matrix and opal. This material will take an incredible polish without using any polishing compounds; in fact, I suggest not using any.

Koroit boulder opal is notorious for having natural pits and vugs, and polishing compounds could fill in these spaces, giving an unwanted look and finish to your cabochon. The more ironstone matrix you have in your working material, the more of a mirror polish you can expect in your final product.

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