Wow!! This is an odd puppy. From one angle it looks like a geofact. The next angle a nutting stone, next angle a pestle, then a grooved maul. All I can say is I would have picked it up also. Good luck on finding out what you have.
First...was it found with other Native American artifacts?
Second .. from the photos I see only mother nature at work.
Its doesn't show any evidence that a human used a tool to create any of the shapes or marks on that stone. Best of luck though.