Paul B. Downing in his book, Opal Adventures (published in 1993) talks about finding opal in Andamooka, Australia.
“The opal formation at Andamooka is different from any other. The opal bearing areas are covered with a sandy clay called kopi (kaolinized sandy clay). Mixed in this area are boulders of quartzite (which miners call river rock). Occasionally when these quartzite boulders are near the opal level they will have lines of opal filling cracks in them. (These are called painted ladies.)
Just above the opal level, the kopi and boulders are sometimes joined by another unusual formation called concrete. The sand has been stuck together by opal. It is hard to cut, but very porous and light. (It is not very valuable as a jewelry stone, but when treated to turn black, it can produce an attractive play of color.)
The opal level is the point where the kopi meets a layer of mud clay. The Mines Department claims that nearly all opal is found within 0.2 meters vertically of this meeting place. This narrows the miner’s search.
The solid opal forms in a round shape that miners call a blob. Technically, it is called a lens. Only occasionally are seams of opal found, as well as matrix opal.”
The opal in this area is mined by the shaft and tunnel system.
If you have precious opal atop either common opal or another substance, is it considered a doublet? No – not if they are formed naturally together.
Natural opal – natural precious opal cut to include common opal or potch in one piece
boulder opal - opal cut to include ironstone, rather than common opal or potch)
Doublet – A natural opal assembled with common opal, potch, or any other material
Triplet - the same.
Some dealers attempt to sell assembled opal and ironstone as boulder opal – don’t be fooled!
A wavy line between the two indicates a natural boulder opal. A straight line = an assembled boulder opal, i.e., a doublet.
The fields of boulder opal extend throughout most of southern Queensland, from Hubgerford to Winton. Most of the mining activity occurs around Quilpie, in the south, and Opalton, near the center of Queensland.
Boulder opal is famous for its color intensity. No othr opal type can match the size and vibrancy of the color spots that are visible when the piece is held at angles to catch the play of fire.
In adition, the three-dimensions of boulder opal make it easier for jewelry designers to do some unique things with their jewelry. Matched earrings are particularly popular. “Splits” – a boulder opal split in two so that one half is a mirror image of the other half, are as rare as matched emeralds, rubies or colored diamonds…and their prices reflect this.
Boulder opal is also renowned for its durability. Because water content of boulder opal is low, it almost never cracks or crazes as the years go by.
