This is a Large Rough Opal Specimen in its natural state, from the Coober Pedy mines of Australia.
It is 40mm by 25mm, 19mm thick and of 134.7 carats.
I have left this opal as found, but polished two sides to show how the opal layers have been formed. In this case there are three clear bands of colour with smaller amounts elsewhere.
The opal could be cut to shape, but I think displays best as an unusual mineral sample.