It's a quartzite cobble. You will find them on or off sites,etc. We find them on sites here, yet aren't indigenous to the area, which makes me believe that at least some of them did serve a useful purpose. Perhaps as cooking stones, pottery tools, game pieces, etc. Who knows. The ones I have seen show really no heavy useage or modification, so they most likely weren't used in a harsh fashion. However, you will find hammerstones made of quartzite occasionally...they work great!