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.