Thus, it forms a little differently to its interstate brothers and sisters rather than silica rich water becoming trapped in faults in the earth, boulder opal forms within the concretion of the ironstone itself. The result is the stunning patterns and displays of colour we see in opal.īoulder 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 ( ). When the light passes through the spheres and their gaps, it splits like a rainbow from a prism. These deposits are spherical, and it is their stacking- layer upon layer-that creates gaps between the spheres. 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. 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. Opal Matrix: opal formed in the infillings of pores or holes between grains of the host rock in which it was formed. Koriot opal/ Yowah nut: precious opal formed in siliceous ironstone nodules and ripples of colour.
However, due to the difference in how boulder opal is formed, there are a variety unique boulder opal patterns that are highly prized, such as: These pieces can fetch up to $15000 USD per gram.īoulder opal forms brightest and most valuably within cracks and fissures in these boulders. This is why Boulder opal is the second most valuable type of opal.ĭue 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.
#Boulder opal free#
This reduces the value of most boulder opals, making boulder opal free of ironstone very rare and highly valued. The ironstone in boulder opal is often visible on the surface, often effecting the overall appearance of the opal.
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 stones are more commonly cut in freeform shape to best showcase the veins of colour to be on top of the stone.
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