clast of rock (between 4–64 mm in diameter according to Wentworth-Udden scale)
A pebble is a piece of rock with a particle size of 4 to 64 millimetres. They are often made of flint.
Pebbles are larger than granules (2 to 4 millimetres diameter) and smaller than cobbles (64 to 256 millimetres diameter). A rock made predominantly of pebbles is termed a conglomerate. Pebble tools are among the earliest known man-made artifacts, dating from the Palaeolithic period of human history.
A beach composed chiefly of surface pebbles is commonly termed a shingle beach. This type of beach resists wave erosion, and has ecological niches which can provide habitat for animals and plants.