An Australian boulder matrix opal is a naturally occuring precious opal mixed with the parent rock ( ironstone)
rather than having been deposited in seams or patches, like in boulder opal.
The parent rock is a ferruginous rock ( containing iron ), and is called ironstone.
It is found exclusively in Queensland, Australia, and does not need to be treated before cutting or polishing.
The word boulder is used in its name, because it is always found in proximity or among deposits boulder opal, and the mother rock is the same as boulder opal.
It is not to be confused with Andamoonka matrix from South Australia, of which the mother rock is of a different composition and must be treated before cutting and polishing to accentuate the opal colours
When cut and polished, by world standards, boulder opal matrix is a "Natural opal,Type 3.