What is Australian pipe opal ?

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.
It can be cut with leaving a thin layer of sandstone on the back on of the opal .
When cut and polished, by world standards, It is a "Natural opal,Type 1".