What is an Australian boulder opal ?

Australian boulder opal is exclusively an Australian precious opal found in cracks or cavities in an ferruginous  (containing iron) rock, called ironstone.
It is only found in Queensland, Australia.
Boulder opal is cut with leaving a thin layer of the mother rock on the back of the opal seam, which makes it the most stable opal in the world.
It is a very versatile opal as large boulder opals with little opal in are carved into very artistic pieces.
When cut and polished, by world standards, it is a "Natural opal,Type 2".