Gravel (largest fragment in this photo is about 4 cm).

Gravel piles at a roadside, waiting for the road repair gang

Gravel is small, loose, rock particles. It is any loose rock that is larger than 2 mm (0.079 in) in its smallest dimension (about 1/12 of an inch) and no more than 64 mm (2.5 in).

Small gravel is composed of granules (>2 to 4 mm/0.079 to 0.16 in). Large gravel is called pebbles (>4 to 64 mm/0.16 to 2.5 in).

Particles smaller than gravel are sand, larger particles are cobbles.