FAQ: What is a boulder opal? What colour are boulder opals? Where are boulder opals mined? What is the value of a boulder opal?
Australian Boulder opals are the second most valuable type of opal (following black opal). Boulder opals are easily distinguished by their layer of solid brown ironstone left on the back of the stone. Boulder opals, as the name suggests, are mined from large ironstone boulders under the ground. Thin veins of colourful opal forms in cracks and fissures in these boulders. Because these veins of colour are so thin, opal cutters need to leave the ironstone on the back of the opal to form a full sized stone. The thin layer of opal in boulder opals can display any colour of the spectrum in a beautiful play of colour.
Although the ironstone layer providing the backing to boulder opals may appear similar to doublets upon first inspection, the formation is completely natural. The following photographs show a boulder opal from the front, back, and side:
Due to the dark backing provided by the ironstone, boulder opals generally have a dark body tone which leads to a vibrancy of colour similar to that found in black opals. Sometimes the ironstone is visible on the surface of the stone, which leads to a spotted or motley appearance, and thus reduces the value of the stone. A ‘clean faced’ boulder opal, in which a whole bar of colour is visible on the surface of the stone, is much more desirable and valuable than a surface displaying ironstone spots.
The thinness of the bar of colour in boulder opals means that a high cabochoned surface is extremely rare. Boulder opals usually have a flat surface or an undulating surface. Boulder opals are almost always cut in a freeform shape for this reason, to maximise the size of the stone.
Examples of Boulder Opal
Valuing boulder opal per carat is not generally accepted in the industry due to the large component of ironstone present in the stone. However, to give a rough indication, top quality boulder opal may fetch prices of up to AUD $3,500 per carat for a gem quality stone. A clean faced stone with no ironstone spots is generally more valuable than a spotted stone. Read more about valuation of opals.
Boulder opals are found only in the state of Queensland, Australia. Major opal fields in this area include Quilpie and Winton. Find out more about these places in our article about the Australian opal fields.