Boulder Opal is found in the state of Queensland, Australia and is the second most precious type of opal after black opal. The process of mining the rock and then turning it into a polished stone is explained here, how to Split Boulder Opal.
Boulder Opal is formed in Ironstone and in veins of colour bars. This Ironstone can be sandy in composition to deep consolidated dark brown ironstone. Boulder Opal can be found around Yowah, Australia and is more deep brown in colour compared to the sandy ironstone around Winton to Quilpie in Queensland.
This rough boulder opal is also formed in rocks and pancake shaped rocks that need to be split open so that they can be cut and polished into stones. Most opal miners crack open a rock to expose any colour so that they can ask a greater price for the rock. It’s worth noting that not all rocks have colour. It takes a lot of work to mine the rock in the first place and then sometimes miners can crack open a rock with a hammer and find no colour. It can sometimes be a game of luck for the miners. Once colour is found the miner will ‘slab’ the opal bearing rock.