This is a natural opal pipe specimen from the jundah mine in Queensland .As the sand is hard to polish this has been stabilized.
PIPE OPAL -Is crystal opal from unique pipe like structers that is usually formed in sandstone and not ironstone.. The pipes may be several centimetres in diameter and can be hollow or opal filled.They can be found in knotted masses.There formation is possibly joint controlled or they may be from zones high permeability with in the sand stone that acted as a channel for water flow. This stone has been stabilized as the sandstone is hard to polish.
Good luster on the stone