Boulder Black Opal is a special type of opal, found mostly in central Queensland and Lightning Ridge, New South Wales in Australia.

Boulder Black Opal is a curious form of silica which has filled the cracks and crevices in light and dark brown ironstone boulders.

Invariably, the boulder opal stones are cut with the natural host rock (ironstone) left on the back.

Sometimes, owing to the thinness of the opal seams, the boulders are sawn into baroque shapes, and polished with the brown host rock retained around the precious heart of opal. Small boulders, sometimes containing 'kernals' of opal are known as 'Yowah Nuts' after the opal mining town of Yowah.

Boulder 'matrix' opal is a peculiar formation where flecks of rich, flashing colours of opal are scattered throughout the brown ironstone, like twinkling neon lights of a distant city.