The opal fields of Australia were formed at the bottom of a large inland sea. In this sea there were all sorts of marine creatures including turtles, crocodiles, dinosaurs, all sorts of fish and of course shellfish. As these creatures died their bodies sank to the bottom of the sea and were covered with sand and silt. Over millions of years the sea dried up and various clays and sand were deposited over the sea bed. The bodies of the sea creatures obviously rotted away and the bones and shells, being made of calcium carbonate, were dissolved by acids in the soils. This would have left cavities in the shape of the dissolved creatures. Silica then seeped through fissures and cracks and filled these cavities which acted as casts for the silica that filled them. In some cases the silica turned into opal and the reult was opal in the exact shape of the shell or bone that had been deposited on the sea bed so long ago.