This is a loose/unmounted natural opal from the Coober Pedy mines of Australia.
It is roughly triangular in shape, 21mm by 17mm, 3mm thick and of 5.70 carats. This opal has a pale grey background colour with pin-fire flashes of green, orange and yellow across its entire surface. It is polished on one side, with the reverse in its natural state, showing how the opal layer has formed on the bedrock.
This opal could be set to produce an attractive pendant or just displayed as an unusual mineral sample.