The formation of black opal is a one in a billion miracle that is still debated to this day.
It began 110 million years ago during the Cretaceous period when there was an inland sea in Australia created by tectonic movements that rose the global sea level to a peak that then rapidly dropped. As the sea level decreased thick layers of heavy sediment were deposited around Lightning Ridge, which lies on the edge of this ancient inland sea. These sediments were rich in carbon, silica, and manganese, which are the three minerals that constitute black opal.
As time passed a unique set of geological events caused the minerals to leech out of the sediment and form within faults in the rock structure in the clay levels underground. Much of the opal that formed is referred to as common opal, colourless nodules, however in some cases the silica spheres that came from the silica rich solutions aligned in perfect shape and size to allow precious opal to form. It is believed it takes 5 million years for a rough opal to form that contains 1 centimeter thickness of silica.