Boulder Opal
A Boulder opal is attached to the parent rock, ironstone. When the opal is mixed in seams throughout the gemstone, it is generally called a Yowah Nut. While ironstone is generally found in boulder opals, there are other kinds of boulder opals, such as quartzite, rhyolite, and basalt.
When an opal is mixed through the parent rock rather than in seams and patches, it's called a matrix opal. Matrix opals are sometimes dyed by treating them with sugar and acid to give them a black background. These are known as a treated matrix opal and are considered synthetic. Unfortunately, the only way to tell a treated matrix opal from a natural matrix opal is to send it to a lab or break the opal as the treatment only permeates the top of the opal. Most prized boulder opals come from Queensland, Australia or the Andamooka, Australia area.
