What is an Australian boulder opal matrix ?

An Australian boulder matrix opal is precious opal mixed with the parent rock ( ironstone)

rather than having been deposited in seams or patches, like boulder opal.
The parent rock is a ferruginous rock ( containing iron ), and is called ironstone.
It is found exclusively in Queensland.
When cut and polished, by world standards, It is a "Natural opal,Type 3".

Not to be confused with Andamooka matrix from South Australia, which has to be treated to bring out the colours before polishing.