A pebble is a clast of rock with a particle size of 4 to 64 millimeters based on the Krumbein phi scale of sedimentology. It is generally described to be larger than gravel and smaller than cobble. 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, which has armoring characteristics with respect to wave erosion; moreover, this type of beach has unusual ecological niches that may provide habitat for a number of rare and endangered species.
Pebbles can come in a variety of different colors and textures, and can often have streaks of quartz and different colored sedimentary rock. Pebbles are mostly smooth, but dependent of how frequent they come in contact with the sea, can have marks of contact with other pebbles. Pebbles left above the high water mark, may have the growth of organisms such as lichen on them, signifying the lack of contact with sea water.
Pebbles can also be found inland where ancient seas used to be cover the land, when seas retreat the rocks become landlocked. Pebbles can also form in rivers, and travel into estuaries where the smoothing continues in the sea.