Boulder opals were first discovered in a small Queensland ‘town called Blackall in 1869 and became popular in the 1890s when they were introduced in Europe. Boulder Opals are only found in the state of Queensland with opal fields in Quilpie and Winton. Boulder opals are mined from large ironstone boulders under the ground and are easily distinguished by their layer of solid brown ironstone left on the back of the stone. Thin veins of colourful opal forms in ieracks and fissures in these boulders and because these \veins are so thin, opal cutters need to leave the ironstone on the back of the opal to form a full sized stone. The thin layer of opal in boulder opals can display any colour of the spectrum.