This is a loose/unmounted natural opal from the Lightning Ridge mines of Australia.
It is 21mm by 15mm, 3.5mm thick and of 7.95 carats.
This opal is not a gemstone in the true sense, having a black and grey mixed background colour, together with semi-transparent crystal, commonly known as a matrix opal. It is roughly a tear-drop shape, domed and polished on one side, with areas within it which flash blue and green.
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.