Boulder opal is an intriguing precious opal mined in Queensland, Australia. Unlike other types of opal, boulder opal includes some portion of the host rock it grew from, giving each stone a unique character. In addition to having a beautiful appearance and lasting durability, boulder opal also tends to be easy on your wallet. It makes a great gift or addition to any jewellery collection.
Boulder opals exist in various colours and patterns and predominantly come from Queensland, Australia. When a lapidarist obtains a boulder opal specimen, they meticulously examine it to display the precious opal best. Much thought and care go into cutting the stone to ensure the host rock stays undamaged. There are several methods to go about this:
- Cut the host rock to exhibit patches and pockets of opal throughout the ironstone evenly
- Cut the stone to best display the veins of precious opal throughout the gem
- Or cut it, so the opal appears as the surface, and the ironstone looks like a base tone or backing.
Boulder opal is exceptionally resourceful because it minimises waste. Patches and seams that would usually be too small to cut into opal gems are maintained in boulder opal.
As the second most valuable opal type, boulder opal has thin veins. The stones are cut such that the natural ironstone remains as a backing to the opal veins. Being naturally vivid and dark, boulder opal complements your complexion and gives you a fashion statement.
Boulder opal is typically called a natural doublet. This is because the ironstone is left behind the precious opal when cutting for a piece of jewellery. Boulder opal is generally cut in irregular shapes because cutting follows the natural veins in the ironstone.
As the name indicates, boulder opal is typically mined from large ironstone boulders. Boulder opal is relatively small because of the natural veins in fissures and cracks, creating opportunities for colour to be beautifully displayed. It is easily distinguished due to its recognisable layer of solid brown ironstone.
The ironstone layer may indeed look similar to doublets. Boulder opal has a rich dark tone because of the dark backing, but some look different.
The consequence of a dark body tone gives a colour vibrancy not seen in black opals. A clean-faced stone can be more valuable and desirable than a boulder opal with ironstone spots.
Origins: Where Is Boulder Opal Mined?
As we know, most opal is mined in Australia, with boulder opal being no different. While alternative sources of boulder opal include Mexico and Honduras, the most well-known variety is the Australian Boulder Opal, thanks to the beautiful ironstone streaks in the gem. If you want to find boulder opals, head to Queensland, including the towns of Winton and Quilpie. While opals form over millions of years, you can still find opal attached to host rock here. Depending on the geological conditions and location, some opals are attached to sandstone, and others are to ironstone.
Australian boulder opal is easily identifiable due to the host rock and opal mixture. Each location's opals vary in colour, shape, strain, and appearance.
Boulder Opal Formation
Each boulder opal specimen is entirely unique. Each opal's features are a result of how the stone formed. For example, opals from Koroit and Yowah contain an ironstone backing with chocolate tones. Conversely, other boulder opals display a sandstone caramel or light brown colouring.
Over millennia, minuscule layers of silica spheres solidify and grow to form opal. As you might guess from the name, boulder opal comes from significant ironstone boulders beneath the earth's surface. Thin veins of opal develop within the boulder's fractures. When miners find boulder opal specimens, they cut into the host rock rather than the opal veins, preserving the stone's quality.
Boulder Opal Value
Of all the varieties of opal, boulder opal is often the most affordable due to the presence of host rock. It's important not to confuse genuine boulder opals with doublets. An opal doublet usually contains plastic or synthetic backing, whereas the host rock in boulder opals is entirely natural. The lower price point makes them accessible and appealing to buyers. The leading factor in determining value is the quality and quantity of the host ironstone. If a boulder opal contains ironstone predominantly, its value is usually lower because the value sits in the precious opal.
A clean-faced opal with minimal host rock will command a higher price. Like all opals, its value hinges on the gem's quality, source, and appearance. However, it is usually impossible to value this type of opal per carat because of the host ironstone. A clean-faced stone is more valuable and expensive than spotted options.
Caring for Boulder Opal
We've previously written about how to care for opals, and the process is relatively similar for boulder opals. On the Mohs scale, opals are on the softer end of the scale, with a score of 5.5-6. In contrast, because ironstone is very hard, its overall durability increases. That said, you still wouldn't want your stone to receive any hard blows.
To care for your opal, follow these tips:
- Remove opal jewellery before exercise, cleaning, or handwashing
- Store opals separate from harder jewellery 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 jewellery 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 jeweller to ensure the stone's settings are tight and avoid losing the gem.
Few opals are as remarkably 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. Boulder opal is in a class of its own, like the wild and wavy patterns it creates. A rare gemstone only found in Australia and formed by nature; no two pieces will ever be alike.
Ready to find your own boulder opal? Check out our collection here.