Boulder opal is precious opal that forms in ironstone or quartzite matrix. It is found in Western Queensland, Australia and in Andamooka, South Australia. Through sedimentation the opal is deposited in seams, cracks, or voids in the ironstone in the Western Queensland area. In Winton, Australia the opal is contained in small veins or pits in 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. A variety of boulder opal from Andamooka, Australia forms in quartzite and is sometimes called a “painted lady”. The stones can be brown, gray, orange or white. If the stones are dark, they are known as “boulder blacks”. The play of color is caused by diffraction of light off of the organized silica spheres that make up the opal. Boulder opal comes from Andamooka, Carbine, Jundah, Koroit, Kynuna, Mayneside, Opalton, Quilpie, Toompine and Yowah, Australia. Quilpie, Yowah and Winton are the major mining and wholesale locations with Quilpie being the largest producer of boulder opal. Boulder opal has also been reported from Brazil and Canada.
- Optical Properties
- Characteristic Physical Properties
- Chemistry & Crystallography
Boulder Opal Colors
Precious Opal, Natural Opal Type 2, Yowah Opal
Countries of Origin
Canada; Unknown; Brazil; Mexico; Australia
Precious opal was discovered in Australia in 1872. George Cragg discovered boulder opal in 1888 and established the Cragg mine in the Winton district. Due to a drought and harsh conditions most opal mining in the area ceased until the 1960’s when the use of heavy machinery was introduced. The opal in the area is found by scouting certain tree species, magnetic surveying, ground penetrating radar, and gamma-ray logging. Intermitted flooding in the Queensland area since the end of the last century has limited mining in the area. In 1970 Andamooka black opal matrix was discovered at the 17 Mile Field in Coober Pedy. Mining hit its peak in the 1980’s but in 2018 it was reported that fewer than 30 individuals were mining boulder opal.
Normal gentle care. If the material is oiled or stabilized then please avoid heat, chemicals, ultrasonic, and repolishing.
More About Boulder Opal
We have evidence that opals have been around since at least 200 B.C., so they have plenty of folklore that has grown around their beauty. Some we like include the Arabian belief that opals fall from heaven in flashes of lightning; the Oriental belief that opal was the stone of hope; and from the ancient Greeks, the belief that it bestowed its owners with the gift of prophecy.
Yowah Nut is a variety of Boulder Opal that derives its name from a locality in the Australian Outback. The nodules are about walnut size and will have colorful veins running throughout.
- Characteristic Physical properties