The opal fields in Queensland, including Yowah, lie within a belt of deeply weathered Cretaceous sedimentary rocks known as the Winton formation. In this time period there must have been a huge area with vegetation and changing water flows, probably similar to the Amazon in South America. This caused all the deposits of clay and sandstone. These contain the ingredients needed to form opal. The occurrence of precious opal, mainly in siliceous ironstone nodules known as Yowah nuts, make this opal field very special.
Those nodules are basically a natural transformation from clay balls into a ironstone concretion. Most of the time opal is found in combination with the ironstone. This can be "matrix", which is a mix of different patterns of ironstone and opal. This combination can make magnificent picture stones. Secondly, and more rarely, the opal is found in the centre of a nut. This variety makes the Yowah nut very special, and this is the ultimatum for a lot of miners.
Nuts come in all shapes and sizes and can even be segmented. Most of the nuts don't contain any opal at all. It takes an average of about 1000 nuts to get a real god one. From the outside it is not possible to see if there is opal inside, so every nut needs to be split or sawn. Depending on the geological circumstances in the mine, there could be more or less nuts containing opal.
Sometimes a thick layer of sand/ironstone layer with nuts, called conglomerate, can also be found. Opal with a wood structure and called wood replacement, is also available. When the cavity, left behind by a piece of wood, is completely filled with opal it is classified as pipe opal. The carriers for the opal can be found from the surface, in the sedimentary sandstone, down to the clay level. There's no clue to it, but miners believe and say that changes in the ground are needed to form the opal.
Most of the times it is a matter of moving the dirt and having some luck. Mining can be done by tunnelling underground or open cut with heavy machinery. The opal can be found, as in many opal fields, in all different qualities from potch up to black gem crystal. It can be sunflash, hydrophane, white based opal, or even a mixture of those together. The variety is enormous and that why the opal lover will never get bored with a good Yowah nut. Every Yowah nut is beautiful in its own way and is an opal variety with character. Cut stones can be set and are nowadays more and more used in jewellery. Specimens have always been desired amongst opal collectors all over the world.
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