Opalized Bones

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.