Rock found near lake

[Nat006]

Not sure what this rock is, if a mineral or also a fossil. Found near a lake near Odenwald, Germany. Put up a picture of what the sign says about the rocks found there. Thank you. 

 

[Nat006]

This is a picture of the back.

[debivort]

It looks to me like a weathered chert nodule, of geologic origins.

[Nat006]

Thank you