I've ripped off my fair share of pitch and gravel roofs, that's definitely not the kind of stone you'd normally use (but it could still be). When you see granules or rocks on a roof, it's usually meant as a heat sink to stop the tar or shingles from cracking and degrading. Otherwise, I've also ripped off slate roofs, and they used lead wide head roofing nails, though at some point they had just tarred over the whole thing, and eventually we put basic ashpalt shingles on it.