Potch — The Common Opal That Hosts the Precious Stuff
Potch — The Common Opal That Hosts the Precious Stuff
Australian field slang for opal without play-of-colour, ranging from the worthless overburden to the matrix that cradles a nobby
Potch is Australian opal-field slang for common opal — opal that lacks play-of-colour. The term originates in the New South Wales and South Australian opal fields and has been absorbed into international gemmological vocabulary as the standard descriptor for non-phenomenal opal, particularly when found in association with precious opal. Potch ranges in body tone from white through grey to black and brown, can be transparent to opaque, and forms the matrix in which seam and nobby precious opal develop in Lightning Ridge, Coober Pedy, Andamooka, and the smaller fields of Mintabie, White Cliffs, and Yowah.
Composition and formation
Potch shares the composition of all opal — hydrated silica, SiO2·nH2O, with water content typically between 6 and 10 percent. The difference between potch and precious opal is structural rather than chemical: precious opal has uniformly sized, regularly arranged silica spheres that diffract visible light to produce play-of-colour, while in potch the spheres are either irregular in size, disordered in packing, or both, so no diffraction occurs. The two materials commonly grow together, sometimes in the same nodule, and the boundary between them in a single specimen can be sharp or gradational.
Potch and precious opal in the major Australian fields formed during the Cretaceous, in weathered sediments above the Great Artesian Basin, where silica-rich groundwater filled cracks and voids in claystone and sandstone host rock and slowly dehydrated to form the silica gel that hardened into opal.
In the mining trade
For the working opal miner, potch is both raw material and indicator. A seam of potch in the wall of a drive often signals nearby precious opal — the geological conditions that deposited one deposited the other. Black potch is particularly significant at Lightning Ridge, where the dark body tone provides the crucial backdrop that makes black opal's play-of-colour read as vivid against a dark ground. Many of the most valuable Lightning Ridge stones are precious opal layers backed by, or overlying, black potch, with the potch retained in the cut to provide the dark backdrop.
Potch and colour stones — pieces in which thin precious-opal seams sit on a body of potch — are a recognised market category. Where the precious-opal layer is too thin to cut as a solid stone, the potch backing is preserved to give the cut piece structural integrity and to enhance colour contrast. This is distinct from a doublet, in which a thin precious-opal slice is glued to a separate dark backing material.
Cabbed potch and contemporary use
Most potch is discarded as overburden or cut away from the precious material during processing, but attractive specimens occasionally reach the lapidary bench. Translucent grey or honey-coloured potch with even body tone can be cut as cabochons in its own right, and unusual colours — some of the brown and orange Yowah and Koroit potch from Queensland boulder-opal country — find a niche market among studio jewellers. Potch from Welo, Ethiopia, and from Mexican deposits also reaches the cabochon market, although Australian usage of the word potch is more restricted than the global use of common opal.
Identification and care
Potch identifies as opal under standard gemmological tests: refractive index near 1.45, specific gravity 1.98 to 2.20, hardness 5.5 to 6.5 on the Mohs scale, and characteristic infrared absorption from structural water. Without play-of-colour the gemmologist's task is to confirm species and rule out chalcedony, glass, or plastic simulants.
Care follows opal protocols: avoid thermal shock, dry storage that risks dehydration crazing, ultrasonic and steam cleaning, and prolonged contact with solvents or detergents. Mild soap, warm water, and soft cloth are the standard cleaning regime.