Found in South Western and Central Queensland, this beautiful variety of Opal is similar in appearance to Black Opal and forms naturally within ironstone "Boulder" which is cut to form part of the gemstone.
Boulder is a variety of precious opal which has an ironstone host rock forming naturally as part of the gem. Often just a thin vein of precious opal is present.

It occurs in specific locations over a wide area of Queensland where the opal fills cracks or voids in ironstone boulders. Boulder opal can be black, dark or light depending on the appearance of the stone when viewing the presentation face.
Boulder opal comes from the Queensland fields and is often cut and sold joined on to the host boulder rock. Some of the most sensational opal has been formed on boulder opal.
Boulder opal is also classified as solid opal. It occurs as thin veins of precious opal in the cracks and cavities of light and dark brown ironstone boulders in Queensland. The opal flowed into the cracks and fissures in the boulders in liquid form millions of years ago. With the passing of centuries, the liquid material formed into solid opal and now miners cut these stones into magnificent pieces with the natural host rock left on the back.
Boulder opal can be found in many different forms and colours: its surface can be smooth or uneven, with the opal occurring as a solid piece on top of the ironstone or showing as flashing flecks of colour throughout the ironstone (known as matrix opal). There are also the famous nut opals, known as 'Yowah-nuts' and unique to Queensland. These smaller ironstone concretions up to 5cm across may host a kernel of solid opal or contain a network of thin veins of opal through the ironstone. The best development of this variety of opal is at Yowah, hence the name 'Yowah-nuts'.
Boulder opals are fashioned to standard shapes and sizes but are mostly cut in freeform shapes to highlight their individual beauty and to avoid unnecessary wastage.