Its hard to tell without signs of workings or wear marks,but WNCsweeper is right--they used anything in nature to achieve a task. Early Native Amer. didnt really have a need for a handle with crude era tools,most hand tools were rounded or small,to fit in hand easier,to do small tasks, big heavy non-specific rock tools were not easy to carry,something like that would have been used at the time time of need and discarded.--so stone age tool prob not--picked up and used by a native to crack a nut apart for a dinner around a fire -why not?
it is also important to remember Native Americans were not "primitive" in their tools or behavior. They produced very fine artifacts to do the most mundane tasks...what you have is a rock..post this on the NA artifacts thread and you will hear the same thing.