Creation
Opals are actually silica gel that have filled gaps in rock formations and then hardened. When this silica gel seeps into gaps in rocks that have been made by fossils, the opal will essentially create a mold of the previously organic material.
Geography
Opalized fossils are not abundant but can be found in small quantities all over the world. Southern Australia, for example, has a large deposit of opalized fossils, including clam shells, in Coober Pedy.
Uses
Like other types of opal, opalized clam shells are used primarily in jewelry and art. Working with shells requires experience, and the pieces need to be polished to show the rainbow coloring of opals. Because of their rarity, opalized shells are usually more expensive than their more common opal counterparts.