Boulder opal rough is opal taken straight from the mine. It requires some skillful cutting and polishing to remove the excess ironstone and a little luck to split or slice in the right place.
So far this type of opal comes exclusively from Australia and the Queensland fields in particular’. It has a dark to black body tone as defined by the body tone scale for opal classification. Actually, this is another form of black opal because of its dark opaque appearance
Boulder opal comes in brown hard ironstone
Boulder opal rough is opal still attached to the mother rock, in this case, ironstone. The opal mostly runs in seams inside the ironstone and is often unseen until the opal cutter makes a cut into the rock.
Some boulder opal is discovered by cracking open a piece of ironstone boulder to see whats inside. The opal seam sometimes splits the boulder in half and this can often result in mirror pairs of opals called ‘split faces’. These are often cut and shaped to be used as matching earrings.
Matrix is another kind of opal. This stone, rather than appearing as seams in the ironstone shows up as flecks of opal color scattered through a piece of ironstone. This type of boulder opal rough is often used for making beads which look spectacular.
Cutting and Polishing this unique opal
Cutting and polishing this stone is a specialized form of opal cutting. It can be a little messy because the boulder ironstone can stain your clothes if you are not careful so it’s good to have appropriate spray control and select machinery that allows the water to drain away rather than run in a muddy ‘soup’ as was the old way of doing it.
The rough boulder opals are cut using a diamond blade and then cleaned up and shaped on an opal grinding wheel. Polishing is done with sandpapers and a leather lap or felt wheel with cerium oxide paste to get the final polish. Rubberized diamond wheels are also used very successfully for boulder opal cutting but more details of this can be found in our section on opal cutting.
Many collectors keep polished and unpolished boulder opals as specimens or conversation pieces. The dark brown ironstone is a fantastic setting to display opal in and many jewelry stores that have opal jewelry will have a specimen to drape their jewelry over because it’s a great way to see the product from its original state to being able to be worn.