This is a shaped and polished freeform, ready to mount or display, natural Lightning Ridge opal from the mines of Australia.
It is an irregular shape, approx. 20mm by 17mm, 4mm thick and of 12.50 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 black and grey striped backing stone with opal bands which flash mainly red, with some green.
A totally unique stone, which would make an unusual ring/pendant or simply kept as a mineral sample.