Boulder Opal is a unique type of opal found primarily in Australia. It is characterized by its vibrant colors and patterns, which are formed within a host rock, usually ironstone. Unlike other opals that are often mined as pure gemstones, Boulder Opal retains a portion of the surrounding rock, giving it a distinctive appearance. The colors in Boulder Opal can range from blues and greens to reds and yellows, often displaying a play-of-color effect. This phenomenon occurs when light interacts with the microscopic silica spheres within the opal. Due to its natural beauty and rarity, Boulder Opal is highly sought after by collectors and jewelry makers.