A pebble is slang for a tity underneath the sun.
Person A (at a nude beach): Wow, she's got a nice pebble!
Person B (also at a nude beach): Damn, she really does!
A tiny ass rock that gets lodged under a skateboarders wheel causing him/her to fall like a poser.
"check dis out gurls, it's called a bs 360 heelflip *begins trick, hits pebble and falls to ground* fuck, nevermind."