The stunning Australian boulder opal is a unique and rare stone that portrays a vast array of colours. Boulder opal is found within large boulders, which are mined out of the harsh outback throughout Queensland, Australia. This is why the stone is commonly referred to a Queensland (QLD) boulder opal.
Boulder opal was first discovered in the small town of Blackall in central Queensland, Australia in 1869. Over the following two decades many discoveries were found however value in the stone wasn’t gained until it was introduced into the European market around 1890. This is when Queensland boulder opal gained its reputation as a gem quality stone, now rivaling Lightning Ridge black opal.
Koroit boulder opal (cracked open)
There are several opal fields in Queensland where boulder opal is found. The Winton Formations, where the opal fields are located, are a belt of deeply weathered cretaceous sedimentary rocks that are approximately 300km wide. Lying in a north westerly direction, the Winton Formations cover approximately 1000km, stretching from the New South Wales border at Hungerford all the way to the west of Cunnamulla, Longreach, Quilpie, Winton and Kynunga.
Map of the mining fields in Australia
Boulder opal pendant set in 9ct gold
Boulder opal is widely distributed throughout the Winton Formations. The opal is enclosed within boulders that can range from a few centimetres up to 3m across. Depending on the location, the host boulders may be confined to one layer of earth or randomly distributed throughout the weathered sandstone ground. The composition of the host boulder ranges from ironstone to sandstone. Ironstone boulders are composed almost entirely of iron oxides whereas sandstone boulders have a rim or crust of ferruginised sandstone surrounding a sandstone core.
The virgin host boulders are usually mined using open cut methods involving heavy machinery. The machinery removes vast amounts of dirt and rock which uncovers the precious opal bearing boulders beneath the surface. Open cut mining is vastly different from shaft mining which is seen in other opal fields throughout Australia.
Depending on the host rock, the formation of the opal can vary. Yowah nuts, boulder matrix, seam and pipe opal are different examples of this.
Yowah nuts are generally small ironstone boulders ranging up to approximately 5cm across that contain a centre kernel of either solid opal or a network of thin veins running throughout the host rock. It gets its name from the location where it is found, the region of Yowah, Queensland, Australia.
Rough Yowah nut
Yowah nut cracked open to reveal opal
Yowah center kernel pollished
Boulder matrix opal is where the opal forms a network of veins throughout the host rock as a result of infilling of voids within the boulder. Boulder matrix opal can be found within both ferruginous sandstone and ironstone host rock. Boulder matrix opal should not be confused with Andamooka matrix (see our blog on Andamooka matrix for more info).
Qld boulder opal showing opal veins running through sandstone host
Seam boulder opal is a rare formation where the opal occurs as a filling between the concentric layers of the host rock, most commonly ironstone.
Qld boulder opal (potch on ironstone)
Pipe opal occurs within sandstone host rock as pipe like structures, hence its name. Pipe opal can be up to several cm in diameter, hollow or opal filled.
As well as how the opal is formed, the actual type of opal contained within the formation can vary as well.
Black boulder opal is very rare and valuable. The uniqueness of this type of opal is in the uniform darkness of its black opal colour and complete colour spectrum that can be seen within. It rivals Lightning Ridge black opal.
Crystal boulder opal is given its name by the transparency of the opal. These translucent opals are highlighted by the dark host of ironstone or ferriginous sandstone that can seen behind the polished surface of the opal.
Light boulder opal is similar to crystal boulder opal except that the opal is not completely transparent and contains some opacity.
Qld boulder opal
Cutting boulder opal can be challenging. The processes involved are different to other types of opal. Boulder opal is most commonly kept with the host rock forming part of the precious stone. Boulder opal is usually found as a thin vein and the face of the opal is undulating conforming to the voids and seams within the host rock. Boulder opal is difficult to cut on a wheel and often requires hand carving to display the intricate pattern of the opal. Boulder opals can be cut into standard shapes and sizes but are often cut in freeform shapes to highlight its natural beauty.
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