Around 400 stone balls have been found in Scotland. Around half of those have no decoration and are sort of "half finished". The finished ones can be incredibly ornate and have all sorts of geometric patterns and ornate knobs and projections.
http://www.hunterian.gla.ac.uk/collections/museum/online_exhibitions/stones/carvedStoneBall.jpg
They tend not to be found in the context of stone circles but in ploughfields. I think the exceptions were these incredible pieces found in the houses at Skara Brae.
http://feralstrumpet.files.wordpress.com/2007/10/carved_stone_objects_skara_brae.jpg?w=535
Ball bearings? Research Grant? Unbelievable.