This is a loose/unmounted large natural specimen crystal opal from the Coober Pedy mines of Australia. It is 27mm by 16mm, 7mm thick and of 19.05 carats.
This opal is not a gemstone in the true sense, being a matrix crystal opal with inclusions. The opal is oval shaped, domed and polished on both sides. It has a grey semi-transparent background colour, with flashes of blue and green across its entire surface. Throughout the opal are small black spots, which is reminiscent of a birds egg.
This opal could be set to produce a stunning pendant or just displayed as an unusual mineral sample.