Boulder opal consists of concretions and fracture fillings in a dark siliceous ironstone matrix.
It is found sporadically in western Queensland, from Kynuna in the north, to Yowah and Koroit in the south. Its largest quantities are found around Jundah and Quilpie (known as the “home of the boulder opal”) in South West Queensland. Australia also has opalised fossil remains, including dinosaur bones in New South Wales, and marine creatures in South Australia. The rarest type of Australian opal is “pipe” opal, closely related to boulder opal, which forms in sandstone with some iron oxide content, usually as fossilized tree roots.