About the genuine Australian boulder opal stone:

Genuine boulder opal stone is a precious opal only found in Queensland, Australia.
Despite claims made by some opal merchants, any so-called boulder opal found in any other country than Australia, is not a true boulder opal stone.
Australian boulder opal stone is found in cracks or cavities of various thickness, in an ferruginous (containing iron) rock, called ironstone.
Australian boulder opal stone is cut and polished leaving a thin layer of the mother rock on the back of the opal seam.
It is the most stable opal stone in the world.
It is a very versatile opal: large boulder opal stones with little opal in it, are carved into very artistic pieces.
When cut and polished, by world standards, it is a "Natural opal,Type 2".