Stone with dent in it perfect for thumb, what is it?

[Nat006]

Probably just a rock but I want to make sure. What could've formed the dent at the bottom? It seems very natural/smooth compared to the broken part at the top.

 

Found in Germany, 

[Kane]

Likely weathering. I think I can see the initial break as evidenced by the hackle fringe in the divot area. The break at the top may simply be more recent and was not given the opportunity to weather yet.