light opal
crystal opal
boulder opal
boulder matrix opal
black opal
composite opal
Boulder opal is found in select areas of inland Queensland. The term boulder comes from the ironstone on which the layer of opal is formed, often giving the opal an uneven surface to further enhance the array of colours.

Opal is formed in narrow cavities of the ironstone, when split they often form what are called 'boulder pairs'.