These are clearly characters from studio ghibli's take on Night at the Museum.
So if I understand correct, there's some other base substance inside?
Yes, kinda. Sometimes it's just a small bit where crystals nucleated. Sometimes there are fossils of (generally shelled) creatures or bits of creatures. Commonly stuff like clams, shrimp, maybe crabs, other molluscs. Sometimes intact but the material is similar to the things you see washed up on a modern beach.
Also the ones displayed are unlikely to have fossils in them. You can sometimes tell by the shape of the concretion.
Interesting! What do you think if any is inside these ones?
You have to choose which one to bring back as a pokemon.
Team ROCKet's blasting off again...