Can you use a beach pebble as a pumice stone?

It’s probably safe, but pebbles (at least when I think of it) are normally small and smooth; pumice is a little bit rough, and that’s what makes it work.

Just walking on the beach will exfoliate your feet. If you rub the wet sand on your knees & elbows, you’ll naturally exfoliate.

Sure, if you find a big enough piece of pumice. Do you live near a volcano?

As inexpensive as pumice is, you would be just as well off investing a couple dollars. They generally are also equipped with handles, so there’s that too.