How are opal fossils formed?
Opal fossils are formed mainly in two major opal fields: Coober Pedy and the Lightning Ridge/Coocoran fields. These areas produce a higher concentration of opalised fossils because silica‑rich water fills old cavities left behind by ancient shells, plants, and bones that have eroded away.
The most common opal fossils include mussel shells, pipi shells, yabby buttons, and plant material.
Rarer opal fossils include dinosaur bones, teeth, and belemnite fossils. One famous example is Eric the Plesiosaur, discovered in 1987 at Coober Pedy — the most complete opalised vertebrate fossil ever found.
Some opal fossils even display bright colour and pattern. Opalised belemnites are a great example — including the world‑famous ‘Virgin Rainbow’ opal.
Theropod Tooth – Lightning Ridge, NSW
Fish Jaw Bone – Coober Pedy, SA
Pipi Shell – Lightning Ridge, NSW
Check out this video explaining opalisation and showcasing real opal fossils found in Australia: