This is a loose/unmounted natural opal from the Coober Pedy mines of Australia.
It is 19mm by 8mm, 6mm thick and of 11.00 carats.
This opal is not a gemstone in the true sense, having a semi-transparent grey background colour with white and black inclusions. The opal is pear-shaped, domed and polished on both sides. One face flashes blue and green with the other having a more purple colour with just blue flashes.
This opal could be set to produce an interesting ring/pendant or just displayed as an unusual mineral sample.