Boulder opals are only found in Queensland, Australia. Primary opal fields include Winton, Opalton, Quilpie, Yowah, Eromanga, Cunnamulla, and Longreach.
They are easily distinguished by their layer of solid brown ironstone left on the back of the stone. As the name suggests, this variation of opal is mined from large ironstone boulders under the ground. Unlike other forms of opal, it is often mined via open cut mining technique using an excavator.
The stones are more commonly cut in freeform shape to best showcase the veins of colour to be on top of the stone. The ironstone backing is what makes the opal naturally vivid and the colour so vibrant, creating what is often referred to as being a natural doublet.
The ironstone in boulder opal is often visible on the surface, often effecting the overall appearance and value of the opal; making opal free of ironstone very rare and highly valued.
Due to the significant weight different in iron stone vs sandstone, valuing this type of opal per carat is not common practice and not a proper indicator of a boulder opal’s value unless ironstone is not present in the piece. These pieces can fetch up to $15000 USD per gram.
Boulder opal forms brightest and most valuably within cracks and fissures in these boulders. However, due to the difference in how the opal is formed, there are a variety unique opal patterns that are highly prized, such as:
Koriot opal/ Yowah nut: precious opal formed in siliceous ironstone nodules and ripples of colour.
Opal Matrix: opal formed in the infillings of pores or holes between grains of the host rock in which it was formed.
Matrix opal is also formed in Andamooka. However, Andamooka matrix opal is formed within limestone. Andamooka matrix opal is regularly treated to darken the colour of the host limestone creating a darker host rock, therefore highlighting the sparkles of opal within.
Of all gemstones, when opal was formed through cervices in the earth filling with silica rich water 100 million years ago, silica deposits were left behind. These deposits are spherical, and it is their stacking— layer upon layer—that creates gaps between the spheres. When the light passes through the spheres and their gaps, it splits like a rainbow from a prism. The result is the stunning patterns and displays of colour we see in opal.
Boulder Opal is unique to the eastern shore of the ancient inland sea with vibrant colour being found in seams which can be cleaved apart on their natural fault lines to reveal faces of opal ( https://gem-a.com/gem-hub/gem-knowledge/understanding-boulder-opal ). Thus, it forms a little differently to its interstate brothers and sisters; rather than silica rich water becoming trapped in faults in the earth, the opal instead forms within the concretion of the ironstone itself. This process very commonly forms thin veins of colour.
Because of this, opal cutters need to leave the ironstone on the back of the opal to form a full sized stone. The thin layer of opal the ironstone can display any colour of the spectrum.
Shop these opals in store at https://www.goodopalco.com/product-category/opals/boulder-opal/.
Boulder Opal Cutting Tips
Cutting Boulder Opal is a little different to cutting other forms of Australian Opal. The hardness of the host rock is quite similar to that of Andamooka Opal. For one, it is much messier to cut (beware, it stains everything) and needs to be approached differently in order to maximise the colour.
Most boulder opal is cut with a flat face because the colour is often very thin. It also can’t pass the light test as its base is completely opaque much like black opal.
So, the best approach to cutting boulder opal is following the veins of colour in the iron stone. To ensure a smooth face without pits stay on the lighter side of grits ie. 600 grit for careful hand and 1200 for an inexperienced one. When it comes to polishing (and this also goes for Andamooka Opal) the better polish for this variation of opal is 50000 grit diamond polishing compound as opposed to cerium oxide.
The reason being is cerium oxide gets into every crevice of the opal when polishing and whilst this works beautifully for all other forms of opal, because the host rock of boulder opal and Andamooka opal is often porous with small pits and lots of crevices, the cerium oxide fills these voids, leaving white dots all over the opal host rock. The Diamond paste compound has a darker appearance and much less noticeable on these variations of opal.