Boulder opal is a distinct variety of precious opal that forms within the cavities, seams, or fissures of ironstone boulders (or occasionally sandstone), primarily in Queensland, Australia. Unlike solid opals, which can be separated from their host rock, boulder opals remain permanently attached to their natural iron-rich matrix—typically a reddish to brown sedimentary rock. This unique structure creates a striking visual contrast between the opal’s vibrant play-of-color and the earthy hu...
