Gravel is rock or stone of a size range more than one half inch but less than two and one half inches in diameter. Natural erosion of big stones leads to the formation of gravel as does the banging of bigger rocks rolling down hills and crashing into smaller rocks. But for masonry, construction and road-building crushed stone and gravel are formed by mechanical crushers. Gravel is also used aesthetically as well as natural stone, pebbles and river pebbles are used in paths and gardens. Pea gravel is found in nature buy can be manufactured as well but requires an extensive tumbling process. If you ever tumbled rocks as a scout, you probably recall that the tumbling and polishing process requires many many hours and days of mechanical stone tumbling.