Boulder Opal

Opal was first discovered in Australia in 1872. Boulder opal is precious opal that forms in ironstone matrix, in Winton, Queensland or quartzite matrix, in Andamooka, South Australia, both locations within the Great Artesian Basin of Australia. In Winton, the opal is deposited in small veins or voids within the ironstone and is known as boulder matrix opal. If the opal forms in a banded pattern or central core the size of a walnut, it is called a Yowah nut. The boulder opal from Andamooka, is sometimes called “painted lady”. Boulder opal matrix may be brown, gray, orange or white. If the matrix is dark, the stones are known as “boulder blacks”. Opal play of color is caused by light diffraction induced by organized silica spheres within the precious opal. Boulder opal locations within the Great Artesian Basin include, Andamooka, Carbine, Jundah, Koroit, Kynuna, Mayneside, Opalton, Quilpie, Toompine and Yowah. Quilpie, Yowah and Winton are the major mining and wholesale locations with Quilpie being the largest producer. Boulder opal has also been reported from Brazil and Canada.