This is a loose/unmounted large natural specimen matrix opal from the Coober Pedy mines of Australia. It is 25mm by 19mm, 4mm thick and of 12.05 carats.
This opal is not a gemstone in the true sense, being a matrix crystal opal with black inclusions. The opal is triangular shaped, domed and polished on both sides. It has a grey semi-transparent background colour, with flashes of blue across its central section.
This opal could be set to produce an unusual and unique pendant or just displayed as an unusual mineral sample.