What is a pipe opal stone?

A pipe opal stone is a precious Australian opal stone having formed in roundish or tubular holes in the mother rock, usually sandstone, but can be also found in boulder.
Most of pipe opal stone found is crystal opal with bright colours.
It is found exclusively in Queensland, Australia.
It is cut with leaving a layer of sandstone attached to the back of the opal stone ( as a layer of ironstone is left on the back of a boulder opal stone).
When cut and polished, by world standards, It is a "Natural opal,Type 1" if no backing is left,
or "Natural opal, Type 2" if some of the mother rock is left on the back of the opal stone.