These opal fields lie within a 300 kilometre wide belt of deeply weathered Cretaceous sedimentary rocks known as the Winton Formation, which extends in a north northwesterly direction from Hungerford on the New South Wales border, west of the townships of Cunnamulla, Quilpie, Longreach and Winton, to Kynuna, a distance of about 1000 kilometres.
Boulder opal is widely distributed in these rocks within ironstone concretions or boulders, which are generally elongated or ellipsoidal in shape (from a few centimetres across to up to 3 metres in size).
The boulders may be confined to one or more layers known as the boulder layer or may be randomly distributed through the weathered sandstone. Their composition ranges from sandstone types (a rim or crust of ferruginised sandstone surrounding a sandstone core) or ironstone types (composed almost entirely of iron oxides).
The opal occurs as a filling or lining between the concentric layers or in radial or random cracks in the ironstone, or as a kernel in smaller concretions or nuts (as found at Yowah and Koroit fields, the famous ‘Yowah-nuts’).
Matrix opal is where the opal occurs as a network of veins or infilling of voids or between grains of the host rock (ferruginous sandstone or ironstone).
Rare seam or band opal is also found and is typically encased in ironstone.
Pipe opal occurs in pipe-like structures which may be up to several centimetres in diameter within the sandstone and these structures may be hollow or opal filled.
Wood opal is occasionally found replacing woody tissue material.
As opposed to other sedimentary precious opal, boulder opal is attached to the ironstone and stones are usually cut with the natural ironstone backing intact. Solid opals may be cut from the ironstone material where the opal is of sufficient thickness.
Boulder opals are fashioned to standard shapes and sizes but are also cut in freeform shapes to highlight their individual beauty and to avoid wastage. Magnificent picture stones are also cut but these are mainly of interest to collectors rather than for jewellery use.