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
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.