The artifact guide published by the Massachusetts Archaeological Society lists "stone ball" among the class of artifacts described as gaming stones. Here's how they describe a stone ball. Everything within the quotation was written by amateur archaeologist William Fowler in 1963. The end of the description was included in the 1991 revision. So, although listed with game stones, there really does not seem to be a proven purpose. That said, I'm guessing when brought to a full polish, it might be tough to distinguish from glacial cobbles, for instance, that have a polish and are spherical.
"Artifacts(exhibits #1 and #3) have been recovered as surface finds, which have been pecked into nearly round balls, relatively heavy, and large in size. They are not water-worn pebbles, for pecked scars are prominent over all surfaces. These balls are evidently man-made, since the pecking has left irregular facets , which prevent the balls from being exactly spherical". So far, nothing is known of their possible utilization. They may possibly have served as large maize-grinding stones or hammerstones rather then for gaming.
I've never seen one from these parts, to my recollection. But here are the two examples referenced in the citation from the MAS artifact guide. Nos 1 and 3. Each is about 3" in diameter: