Boulder Opal

Boulder opal is opal that is attached to the parent rock it is mined from, usually ironstone. When the opal is mixed in seams throughout the stone, it is generally called a Yowah Nut. While ironstone is generally stone that is found in boulder opals, there are other kinds of boulder opals, such as quartzite, rhyolite, and basalt.

When opal is mixed through the parent rock, rather than in seams and patches, it's called matrix opal. Matrix opals are sometimes dyed - treated with sugar and acid - to give them a black background. These are known as treated matrix opals, 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 permiates the top of the opal.