Opals forming in the bones of dinosaurs and plesiosaurs—prehistoric skeletons turned into rainbow-colored gemstones over 100 million years.
An Ancient Sea of Gems
About 120 million years ago, during the Early Cretaceous period, central Australia was not a desert. It was a vast, shallow inland sea called the Eromanga Sea. This epicontinental sea, which covered up to one-third of the continent, was cool, with winter conditions sometimes approaching freezing. It was populated by a variety of marine reptiles. Plesiosaurs and their short-necked cousins, the pliosaurs, were common predators, hunting the sea's abundant fish.
When these creatures died, their skeletons would sometimes sink into the soft mud and clay on the seafloor. Over millions of years, the sea retreated and the climate changed, leaving behind sediments with silica. This allowed for a rare form of fossilization. The process is a type of pseudomorphism, where one mineral replaces another molecule by molecule. In Coober Pedy, silica-rich groundwater percolated through the porous rock, dissolving the original calcium carbonate of the bones and depositing hydrated silica (SiO₂·nH₂O) in their place. This process perfectly preserved the shape of the bones, turning them from organic remains into opal.
Skeletons of Rainbow and Light
The fossils found in Coober Pedy are primarily from marine animals that inhabited the Eromanga Sea. One of the most famous discoveries is "Eric," the opalised skeleton of a small pliosaur, Umoonasaurus demoscyllus, found by a miner in 1987. Now housed in the Australian Museum, Eric is one of the most complete opalised vertebrate skeletons ever found. The discovery of fish vertebrae inside its gut confirmed its diet.
The opalisation process can result in two types of fossil preservation. Sometimes, the original bone or shell rots away completely, leaving a cavity or mold that is later filled with silica, creating an opal cast. This preserves the external shape well. In rarer cases, the silica solution infiltrates the organic material before it decays, replacing the internal cellular structure. This method of replacement can preserve incredibly fine anatomical details, turning an entire skeleton into a solid gemstone. While dinosaur fossils are found in other Australian opal fields like Lightning Ridge, Coober Pedy's treasures are the reptiles that swam in the ancient sea.
💡Fun Facts
The name Coober Pedy is an anglicized version of the Aboriginal term "kupa-piti," which means "white man's hole," referring to the underground dwellings.
A significant portion of Coober Pedy's residents live in underground homes called "dugouts" to escape summer daytime temperatures that can exceed 45°C.
The most complete opalised vertebrate skeleton found in Coober Pedy is a pliosaur nicknamed "Eric," which was purchased for the Australian Museum after a fundraising campaign involving schoolchildren.
The barren, lunar-like landscapes around Coober Pedy have been used as filming locations for movies such as *Mad Max Beyond Thunderdome* and *Pitch Black*.
The opal fields are vast and unfenced, but consist of private mining leases. For safety, visitors should explore via guided tours or visit designated tourist mines, which typically operate daily from around 8:30am to 5:30pm.
Admission
Access to the general fields is open, but tourist mines charge admission for tours, starting from around $16 for an adult. Entry to the museum portion of some attractions is often free.
Accessibility
The terrain is extremely rugged, uneven, and contains thousands of unmarked, deep mining shafts. It is not wheelchair accessible and requires extreme caution when walking.