An ancient flooded world
Around 100 million years ago, during the Cretaceous Period, much of eastern Australia was covered by the Eromanga Sea, a cool, shallow inland ocean. The area now known as Lightning Ridge was a complex of estuaries and floodplains on the sea's edge, covered in forests of Kauri and Podocarp pines. This environment supported a assembly of life. Dinosaurs, including ornithopods like Weewarrasaurus pobeni, walked the land, alongside some of the earliest known monotremes—egg-laying mammals related to the modern platypus. The waters hosted plesiosaurs, crocodiles, turtles, and freshwater mussels.
The ground itself consisted of silica-rich sands and clays, which would become the Griman Creek Formation. As these animals and plants died, their remains were sometimes buried in the soft, sandy sediments. Over millions of years, the sea retreated and the climate changed, beginning a long and slow chemical transformation that would turn bone, shell, and wood into precious opal. Australia is the only place in the world where opalized fossils of land-based animals, including dinosaurs, are found.
The chemistry of gemstone fossils
The formation of opal requires a specific sequence of geological and chemical events. First, the Cretaceous sedimentary rocks underwent a period of deep weathering. This process released enormous quantities of silica (silicon dioxide) from the sandstone into the groundwater. This silica-rich solution then percolated downwards through the porous rock.
The solution filled cavities in the ground, including cracks, seams, and, importantly, the voids left by decayed organic matter like bones and shells. As conditions changed, the water evaporated, leaving behind a silica gel. This gel gradually hardened, with the silica precipitating as microscopic spheres. In common opal, or "potch," these spheres are irregularly sized and arranged. In precious opal, the spheres are uniform in size and stacked in a regular, repeating lattice.
This orderly structure acts like a diffraction grating. As white light passes through the tiny gaps between spheres, it splits into its constituent colors. The size of the spheres determines the color produced: smaller spheres, around 150 nanometers, diffract violet and blue light, while larger spheres of about 350 nanometers produce rare and valuable oranges and reds. The dark body tone of Lightning Ridge's famous black opal comes from trace elements of carbon and iron oxide within the silica structure. This dark background absorbs more light, which makes the diffracted colors appear intense.