This is a shaped and polished freeform, ready to mount or display, natural Lightning Ridge opal from the mines of Australia.
It is roughly oval shaped, 17mm by 12mm, 4mm thick and of 6.10 carats.
This is a matrix opal and not a conventional gemstone and has areas of opal mixed-in with the backing stone. In this case, it has a grey backing stone with black stripes and faint flashes of red and green along their edges.
A totally unique stone, which would make an unusual ring/pendant or simply kept as a mineral sample.