Opal forms in cavities within rocks. If a cavity has formed because a bone, shell or pinecone was buried in the sand or clay that later became the rock, and conditions are right for opal formation, then the opal forms a fossil replica of the original object that was buried.-- Australian Opal Center via evencleveland
1. Lot of four opalized clam shells, found at Bonhams.
2. Opal bone fossils
3. Opalised jaw of Steropodon galmani
and many more here.