Fleroo, I like your research methodology. I would support your theory of non- gastroliths. My guess without seeing them in person would be an artifact associated with an early industrial site of sort. If 222's brother looks at the sites location on an early map of Gibsonia there was probably an old industry located there. Or fill from an early industrial site was dumped at that location. Those same rocks can be found today just outside of Uniontown, PA. on route 51 near Redstone Creek. I have some under my deck that I found when I was young and living in U-town. The site was an old enamel plant that my great grandfather worked at. I have heard that those "rocks" artifacts were used in a grinding, polishing, tumbling type of process. However, I have not researched the industries located at that site in Uniontown. I have also found some near Leesburg Virginia. One way to know for sure would be to take them to a Paleontologist at the Carnegie Museum.
Hunting Success "Nature will teach you but you must spend time in her classroom"