Fossil Clams, at over 5 million years old these are complete with both valves (sides). You can even see some of the fossilized hinge material. It has never been opened so I have no idea if anything, like a pearl, is fossilized inside.
 
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* Photos show all sides.
* Sizes, shapes and colors my vary due to being a natural stone, being hand-carved and/or difference in computer monitors.