To me it looks like naturally formed river rock. That being said, If I found it and needed a tool, I would most certainly use it. I do believe that N/A, Early Americans, "First Settlers", however you want to refer to these survivors as, if they found a rock that was naturally formed and it suits a job they needed it for, yes it is an artifact.
A rock picked up to smash a bone for its marrow is a tool right?
Nice find and good color,