Gravel, aggregate of more or less rounded rock fragments coarser than sand (i.e., more than 2 mm [0.08 inch] in diameter). Gravel is made up of unconsolidated rock fragments. The most common types of rock used in gravel are sandstone, limestone, and basalt. Gravel is an important commercial product, with a number of applications. Many roadways are surfaced with gravel, especially in rural areas where there is little traffic. Globally, far more roads are surfaced with gravel than with concrete or asphalt; Russia alone has over 400,000 km (250,000 mi) of gravel roads. Both sand and small gravel are also important for the manufacture of concrete. Gravel is often produced by quarrying and crushing hard-wearing rocks, such as sandstone, limestone, or basalt.