It IS an sedimentary rock thats been rounded and polished in a moderate energy water environment. Theres a lot of story in the whole rock . Those little crags and pits may be "ichnofossils" which are traces of a fossils cast or its tracks. Thy could be fosils but the entire rock can tell us a whole lot about the environment it passed through from the time its layers were deposited to when you found it.
Theres even a way we can tell how long its been in sunlight and not buried in the earth