Boulder opal is unique to Queensland Australia. It occurs in deposits in weathered sedimentary Cretaceous rocks in the west of the state.
It is found in siliceous ironstone concretions or boulders ranging in size from less than a few centimetres to more than 20 cm.
Concretions up to 5 cm across, known as ‘nuts’, may host a kernel of solid opal or contain a network of thin veins of opal through the ironstone. This variety of opal is prevalent at Yowah where the concretions form distinct bands—the well known ‘Yowah-nuts’.
Major opal mines are:
At Yowah, miners still use hoist n bucket and use picks or jack hammers to dig for opal, most other mines use excavators with huge buckets and large bulldozers like D12s.
Boulder opal rough needs experienced cutters to work the veins that appear in this rough and some miners sell 44 gallon drums that have to be worked.
The veins are tricky to follow and many cutters use saw to cut and split the boulder rough and colours can be spectacular with all colours of the rainbow.
A split is when miner has cut piece boulder rough and sees good vein of colour, he will put nick along this vein colour with his saw blade and the rock will split open revealing the opal colours. Sometimes it looks like both pieces have been polished so splits are popular in making earrings as very hard to get 2 pieces of opal matching in colour.
Ironstone from Koroit has interesting pattern so unique to this region, sometimes no or very little colour but miners call this black n white and patterns are so interesting and good natural look.
Yowah is famous for Yowah nuts, that is nuts formed with colour inside and some of the brightest colours are found in nuts
Quilpie n Winton have some beautiful opals that have full face of colour.These thin colour bars need good cutters with experience and new cutters can easily rub the opal colour off with incorrect opal cutting equipment or sharp diamond wheels so best practise at your local lapidary club before cutting good opal rough