Pebbles are a naturally-occurring map tile found commonly across the surface of the world near rivers and brooks, and occasionally on beaches. They are the same stone type as the substrate; for example, if the first stone layer beneath soil is basalt, then pebbles will be composed of basalt. Pebbles may be found near boulders of the same substrate.
Like boulders, pebbles can be designated for smoothing. Smoothed-out pebbles turn into tiles of that rock type, and can then be engraved, which is useful early on for giving your dwarves happy thoughts as they pass over them.
In adventurer mode, a pebble tile is an unlimited source of small rocks, which can be useful for throwing.