This is a large rough natural matrix boulder opal specimen from the mines of Queensland, Australia.
It is approx. 19mm by 18mm at its widest points, 4mm thick and of 13.25 carats. The opal layer is slightly concave, polished on one side and on the rich brown ironstone backing.
There are small amounts of colour within the opal layer, but it is not really suitable for jewellery and should be considered an unusual mineral sample.