About the genuine Australian boulder opal:

Genuine boulder opal 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.
Australian boulder opal is found in cracks or cavities of various thickness, in an ferruginous (containing iron) rock, called ironstone.
Australian boulder opal is cut with leaving a thin layer of the mother rock on the back of the opal seam.
It is the most stable opal in the world.
It is a very versatile opal as large boulder opals 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".