Pebbles

From DwarfFortressWiki

Pebbles are a naturally occuring map tile found commonnly across the surface of the world near rivers and brooks, and occasionally as beaches. Usually they are found labelled as rock-type pebbles, for example Basalt Pebbles. They can be found near boulders if you look hard enough.

Unlike boulders, pebbles cannot currently be designated for smoothing. In order to smooth a tile with pebbles on it, you must first build a road or other construction on the tile, which will destroy the pebbles and turn the tile into an ordinary tile of the base rock-type.