This is a loose/unmounted natural opal from the Lightning Ridge mines of Australia.
It is 19mm by 16mm, 5mm thick and of 10.90 carats.
This opal is not a gemstone in the true sense, having a grey background colour with white swirls, within which there are flashes of blue and green. From its formation, this looks to be an opal which has infused into an ancient shell. It is roughly pear-shaped, domed and polished on both sides.
A beautiful stone in its own way and totally unique.
This opal could be set to produce an interesting ring/pendant or just displayed as an unusual mineral sample.