Cobble is what a geologist would call a medium-sized water-rounded piece of rock if it was free or embedded naturally in a matrix (like glacial till or sediment). They’d only call it a cobblestone in the context of like, paving a street or driveway or something.
However, I don’t think that makes it wrong for people / other disciplines to call it a cobblestone.
However, I don’t think that makes it wrong for people / other disciplines to call it a cobblestone.