Bounder opals have a solid layer of brown ironstone on the back of the stone.
They originate from large ironstone bounders, hence the name, underground.
Veins of opals form in the cracks and between the fissures on the bounders and
due to being very thin veins the cutter leaves a thin layer of ironstone to help
for a full sized stone.
Bounder opals are mined in Australia, as are most opals, and are considered the
second most valuable opals after black opals.