Pebble

a small, rounded stone, intermediate in size between a cobblestone and gravel. The term is restricted usually to small stones, well rounded by the action of water or glacial action. They are, of course, of all colors and material, corresponding to the ledges of rock of which they were originally fragments. Conglomerate rock is composed of pebbles and cementing material. Agate is known to the jeweler as pebble. Scotch agates are known as Scotch pebbles. A transparent rock crystal, known also as pebble, is ground into lenses for spectacles.