Mined predominantly in the Western Queensland opal fields of Quilpie and Winton, Boulder Opal looks similar to, and displays the same vibrant colour as, black opal, but forms naturally in brown ironstone rock. When a boulder opal is cut, a layer of the host rock remains on the back of the stone. Unlike other opal which can feature a domed, or cabochoned, surface, boulder opal is almost always cut with a flat or undulating face.