Boulder opal is a unique type of opal that forms within ironstone boulders, primarily found in Australia. Unlike other opals, which are often mined from sedimentary deposits, boulder opal retains its natural host rock, giving it a distinctive appearance. The vibrant colors and patterns seen in boulder opal are a result of the silica gel that fills cracks and voids in the ironstone. This gemstone is prized for its striking play-of-color, which can display a range of hues, including blues, greens, and reds. Boulder opal is often cut and polished to showcase its beauty, making it a popular choice for jewelry and decorative items. Its durability and unique characteristics contribute to its appeal among collectors and enthusiasts.