As veterans of the opal industry, we chat a lot about opals here on the blog. Today, we’re tackling one of the most intriguing opals of all: boulder opals. What is the definition of boulder opal?
In this guide, we’re answering that plus four more frequently asked questions about boulder opals. Let’s get straight to the info you need to better understand this fascinating gemstone.
First up? Let’s define boulder opal.
Boulder opal is a unique gemstone that contains pieces of the ironstone hostrock it grew from, along with precious opal. Boulder opals exist in a variety of colors and patterns, and primarily come from Queensland, Australia.
When a lapidary (gem cutter) gets a boulder opal specimen, they meticulously analyze it in order to best display the precious opal. Much care and thought must go into cutting the gem to ensure the host rock stays intact. There are several ways to go about this:
Cut the host rock to evenly display patches and pockets of opal throughout the ironstone
Cut the gem to best display the veins of precious opal threading through the stone
Or cut it so that the opal appears as the surface, and the ironstone looks like a base tone or backing.
Boulder opal is especially resourceful because it minimizes waste. Patches and seams that would otherwise be too small to cut into opal gems are preserved into boulder opal. So, if boulder opal comes igneous host rock, where in the world is it found?
Most opal comes from Australia, and boulder opal is no different. If you want to know where to find boulder opals, head straight to Queensland. Opals form over millions of years, but you can still find opal attached to host rock here. Some opals are attached to sandstone, and others are to ironstone, depending on the geological conditions and location from which it grew.
Of course, there are other sources of boulder opal in Mexico and Honduras. However, the most famous variety is Australian Boulder Opal , thanks to the presence of beautiful ironstone streaks in the gem.
Australian boulder opal is easily identifiable, thanks to the mixture of hostrock and opal. As with most opals, there are countless varieties of this unique specimen — from matrix to colourmine to Yowah.
The most famous boulder opal fields in Queensland are Winton, Opal town, Jundah, Quilpie, Kyabra, Bulgaroo, Koroit, and Yowah. The last two locations are relatively close together and renowned for their unique patterns, cloration, and stylings. Naturally, each location's opals will vary in color, shape, strain, and appearance.
Each boulder opal specimen is completely unique. For example, opals from Koroit and Yowah display dark ironstone backing in chocolate tones. Conversely, other boulder opals display the signature sandstone caramel and light brown coloring. Each opal’s features are a direct result of how the stone formed.
Speaking of which, the formation process is incredible. All opals form over millions of years, but boulder opals have a unique growth and mining process. Over millennia, tiny layers of silica spheres harden and grow to form an opal. As you might gather from the name, they come from huge ironstone boulders beneath the earth’s surface. Veins of opal grow and form in the boulder’s cracks and fractures.
These veins are incredibly thin, and seep into the small openings of the host rock. When miners retrieve the specimens, they have to cut from the host rock because the opal veins are so thin.
If miners cut directly into the opal veins, it would compromise the quality of the stone. While the result is magnificent, the presence of host rock does affect the stone’s overall value.
Of all the varieties of opal available, boulder is undoubtedly the most affordable. That’s because it contains so much host rock. The proportion of precious opal to host rock varies, but altogether, boulder opal is far more affordable because it’s not 100% precious opal.
Take care not to confuse real boulder opals with doublets. An opal doublet often contains plastic or synthetic backing, whereas the hostrock in boulder opals is completely natural.
Getting back to boulder opal value, the low price point is what makes them so accessible and appealing to buyers. The ultimate factor in determining value is the quality and quantity of the ironstone. If a boulder opal contains mostly ironstone, it’s not going to be worth much because the valuable commodity is the precious opal.
If you find a clean-faced opal with minimal ironstone, you can definitely expect to pay a higher price for it. Like all opals, the value hinges on the quality, source, and appearance of the gem.
We’ve previously covered how to care for opals, and the process is relatively similar for boulder opals. See, opals are a soft material with a lower hardness of 5.5-6 on the Mohs scale. By contrast, ironstone is very hard, increasing their overall durability. That said, you don’t want your boulder opal to come into contact with any hard blows.
To best care for your opal, follow these safety and longevity tips:
Remove any opal jewelry before physical activity, cleaning, handwashing, or sports engagements.
Store opals separately from harder items like diamonds or sapphires. Any surface harder than opal can scratch or damage the gem, so it’s best to isolate them in a safe jewelry box or cloth bag.
Clean your boulder opals regularly using a soft bristled brush, cloth, and lukewarm soapy water. Avoid strong detergents or chemicals which can damage the gem.
Schedule routine annual or bi-annual cleaning with your jeweler to ensure the stone’s settings are tight, and avoid losing the gem.
If we could summarize boulder opals with one word, it would be unmatched.
There are millions of opals in the world, but few are as extraordinarily unique as boulder opals. With the juxtaposition of glistening opal and earthy ironstone, you’d be hard-pressed to find a gemstone as distinctive and characteristic as boulder opals.
Ready to mine through the catalog to find the newest addition to your collection?
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