This is a Large Rough Opal Specimen in its natural state, from the Coober Pedy mines of Australia.
It is 50mm by 22mm, 127mm thick and of 386.35 carats.
This is a seam opal which shows how the layers have been formed over millennia. The upper layer is opal silica, but I can see very little colour in it. As such it is a curio rather than a piece of opal that could be cut and polished for gemstones. Priced as such.