I actually used a stone like that once as a fake egg in my chicken coop (encourages the hens to lay in the nest instead of hiding the eggs.)
They are a fairly common,naturally occurring form where I'm at. We have a layer of tumbled quartz and quartzite gravel and cobbles under the area, so for me to walk down a creek collecting "eggs" isn't too difficult, although the near perfectly shaped ones are harder to come by.
That said, these, and "game balls" turn up at native sites often enough to make me think there must be something to it. As a kid, I certainly would have brought something like that home to play with, even though I had all kinds of fancy modern toys. They may have even just been brought home as "art". The last one I saw was being used as decoration out on a patio along with a couple of seashells and a piece of driftwood.
It is the kind of thing that would be fairly easy to make. It's just a matter of finding something close to the shape you want, then tapping the rock against another hard rock to peck away the parts you don't want, then maybe some grinding at the end to smooth and polish. That said, I think if they were being intentionally made, more would turn up at the large camp/village sites than are found. They are rather uncommon.
