Boulder Opal
Reviewed by David van Niekerk FGAA — Fellow of the Gemmological Association of Australia, qualified valuer.
Boulder opal is the opal you can see the landscape in. Bright blues, greens and reds run through dark brown ironstone, the rock it grew inside over millions of years. Each stone keeps a little of that rock, which is why no two ever look alike.
What boulder opal is
Opal formed when silica-rich water seeped into cracks in the ancient rock of inland Australia and slowly hardened. In Queensland those cracks ran through ironstone, so the opal formed as thin veins in the boulder. The veins are usually too thin to cut away on their own, so the cutter leaves the ironstone behind the opal as a natural back.
It is a solid opal. The rock and the opal joined in nature; nothing has been glued on. That is the difference between a boulder opal and a doublet, where a slice of opal is bonded to a backing by a cutter.
Where the opal runs through the rock in tiny flecks rather than one vein, it is called matrix boulder opal. Split boulders can also open into two matching halves, which makes for a naturally matched pair of earrings.
Where it comes from
Boulder opal comes from the Queensland opal fields, a belt of more than 700 km running from around Kynuna in the north to Cunnamulla in the south, with Winton, Quilpie and Yowah its main centres. Queensland is the home of boulder opal. The ironstone is hard, so much of it is mined by machine in open cuts rather than by hand-dug shafts.
What to look for
- Colour: brightness and range — red is traditionally the most prized colour, then orange, then green.
- Pattern: how the colour sits in the rock. Many boulder opals are loved for the picture the opal and ironstone make together.
- Face: the opal should cover the face of the stone well, with ironstone framing it rather than breaking it up.
- Character: the most important factor of all. Choose the stone that speaks to you.
Wearing and care
Boulder opal is one of the more robust opals because the ironstone supports it. Like all opal it is softer than quartz, so take it off for gardening, gym and heavy work. Clean with warm soapy water and a soft brush; avoid ultrasonic cleaners and harsh chemicals. More: How to care for opals.
See boulder opal in the shop
Every boulder opal in our boutique at the Opera House is a one-of-a-kind stone, set in Sydney. Visit us at Opera Quays, or browse the range online.