Just a tidbit! Sometimes the dating and identification of tools of Prehistoric and Post-Prehistoric people in North America is skewed by viewing everything found based on the Clovis people. However, it is now known that there were Pre-Clovis migrations of people to North America and that the possibility exists from somewhat recent finds, that there were Pre-Historic people who were already living in North America before the migrations.
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,
Looks like a polished hardstone celt or axe head to me. Even has an indentation for hafting on one side. Run it by the experts on the Native American relics forum. Congrats! Sub