Australian pipe opal is precious opal having formed in roundish
or tubular holes in the mother rock, usually sandstone, but can be
also found in boulder.
Most of pipe opal 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 ( just as a layer of ironstone is left on the back of a boulder opal).
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.