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Most of us are familiar with opal, the beautiful gem that displays an array of spectral colors. Opal, whether black or white, is commonly found in the field without being attached to any other materials, with the minimal exception of some sandstone. But there are times when opal forms in seams within the host rock (the matrix),creating what is known as boulder opal. The host, or parent rock is usually ironstone, mudstone or sandstone, but it may also be rhyolite, basalt or quartzite.
The most common form of boulder opal is “seam boulder opal” also called “vein boulder opal”, when irregular natural opal seams (from millimeters to centimeters thick), run through the matrix host rock. When this material is cut, the opal layer rests on the ironstone layer, usually creating two visually distinct lines.
Matrix opal forms when very thin natural opal seams are penetrating the entire host rock (matrix) and become a mixture of opal and ironstone, often creating dazzling patterns. That type of opal is further more accurately referred to by as “boulder matrix opal”. They both use the term boulder matrix opal in order to differentiate it from the other matrix opal, found in Andamooka, Australia, which is a fine-grained opal mixed in throughout the entire host rock, instead of seams running through.
All the boulder and boulder matrix opals are found in the state of Queensland, Australia. Several of the various locations produce very distinct patterns, which are identified by the location prefix, such as Yowah opal, or Koroit opal.
Boulder opal was first found in the “arid regions of western Queensland” around1869. The deposits are found in weathered sedimentary rocks from the Cretaceous age (about 100 million years ago). The town of Quilpie is called the “home of boulder opal”, and more opal fields extend up to Opalton to the north.
Opal penetrates the ironstone host matrix, which is commonly a combination of aluminum oxide, silicon dioxide, and iron oxide, and forms in jagged seams. Sometimes the seams are a couple of inches thick, and other times they may be only millimeters thin.
When the precious opal covers the entire face, it is called “full-face boulder opal”, or “clean-face boulder opal.” Although full-faced opals are considered of higher value, some cutters will purposely leave some of the matrix showing on the face of the opal cab so that the buyer knows that the material is natural and not assembled. Slight marks and protrusions of ironstone are fairly typical.
The best-known matrix opal material comes from the Andamooka area in the state of South Australia. This natural matrix opal is a fine-grained opal mixed throughout the entire light-colored limestone matrix
The pinfire of Andamooka opal is very bright, but the matrix is too pale; therefore, it is usually treated to make it look like black opal, which is considered the most valuable opal. There are two types of treatment: In smoke treatment, opals are wrapped in paper or sand and then heated. The other type, carbonization, is carried out by burning the opal with sugar and sulfuric acid. The Andamooka matrix opal material is rather porous, allowing for successful treatment. This matrix opal was originally called “mother opal” by the opal miners, but the term “Andamooka opal” has replaced it over the years.
Boulder Matrix Opal
There are several locations in Queensland that produce boulder matrix opal, and each one is almost identifiable by the formation and patterns in the opal. Frank Leechman gives a great definition of boulder matrix opal as a “dark chocolate brown stone riddled with cracks which show beautiful and brilliant opal colors.”
One of the oldest boulder matrix opal mining locations, and probably the most famous one, is Yowah (pronounced Yáwa), in southern Queensland. The first registered lease of a field there was in 1884. The name, probably of Aboriginal origin ,reflects the nearby Yowah Creek.
Cunnamulla, in the south, and Winton, in the north, are the closest towns to the famous mining areas of Yowah, Koroit and Maynside. Queensland is described as the real Outback, famous for its isolation and remoteness. There are a few year-round miners and many “fossickers”(rock-hounds) that stay in the fields for a few days or weeks in temporary housing, like tents, campers, or old buses.
Mining is labor-intensive in the arid desert. Shafts and tunnels are dug, and automatic hoists are employed to bring the material to the surface. Digging is usually done with picks or power tools such as electric jackhammers. Old claims are re-worked, and tailings are processed again through a rumbler, which is like a barrel tumbler.