Koroit opal is a boulder opal mined near the town of Koroit in the Paroo Shire of south-west Queensland, Australia. It is named after the surrounding opal field, one of the older Queensland producers, with workings dating to the late nineteenth century. Like all boulder opals, Koroit material consists of thin seams and veins of precious opal embedded in or attached to the brown ironstone host rock, and is invariably cut and polished as a unit so that the ironstone forms a permanent natural backing.

What distinguishes Koroit from other Queensland fields, such as Quilpie, Yowah and Winton, is the prevalence of intricate matrix patterns. Many Koroit stones are sold as matrix rather than as solid bars of colour: the opal threads through the ironstone in dendritic, lacework, ribbon or kaleidoscope-like figures, with play-of-colour appearing in scattered pinpricks, narrow veins or discrete pockets within a brown to chocolate-coloured host. Solid colour bars do occur, but the field is best known for its picture-stone effects.

Geology and occurrence

The Koroit field lies within the Eromanga Basin sequence of Cretaceous sediments that hosts most of Queensland's boulder opal. Opal precipitated as a silica gel in fractures and cavities of weathered ironstone concretions and sandstone. Over millions of years the silica matured into precious opal where conditions of pH, temperature and silica concentration permitted ordered sphere packing, generating the diffraction grating responsible for play-of-colour.

Koroit ironstone tends to be hard, dense and uniformly brown, which gives the cut stones strong contrast and a stable cutting medium. Mining is small-scale, conducted by individual operators and family syndicates working open-cut pits and shallow underground drives.

Trade and identification

Boulder opal as a class, including Koroit, is treated as a natural product in laboratory reports. Because the ironstone backing is part of the original rough, Koroit stones are not classified as doublets and do not require the disclosure that bonded composites do. Buyers should still verify that no glue line, dyed sugar treatment or surface coating has been applied; reputable Queensland cutters do none of these.

In the trade, well-figured Koroit matrix is valued for its individuality. Two stones from the same parcel rarely look alike, which makes the material attractive for one-of-a-kind jewellery but difficult to match in pairs or suites. Pricing is driven by the brightness of play-of-colour, the coverage of colour across the face, the balance of the ironstone pattern, and the freedom from sand pockets or potch zones.

Care and setting

Boulder opal is more durable than Lightning Ridge black opal because the ironstone backing reinforces the thin opal layer and reduces the risk of cleavage and crazing. Hardness remains around 5.5 to 6.5 on the Mohs scale, and the stone should still be protected from sharp blows and from prolonged immersion. Heat sources, ultrasonic and steam cleaners are all unsuitable.