Sand is a granular material composed of finely divided mineral particles. Sand has various compositions but is defined by its grain size. Sand grains are smaller than gravel and coarser than silt. However, the exact definition of sand varies. The scientific Unified Soil Classification System used in engineering and geology corresponds to US Standard Sieves and defines sand as particles with a diameter of between 0.074 and 4.75 millimeters. By another definition, in terms of particle size as used by geologists, sand particles range in diameter from 0.0625 mm (or 1⁄16 mm) a volume of approximately 0.00012 cubic millimeters, to 2 mm, a volume of approximately 4.2 cubic millimeters, the difference in volumes being 34,688 measures difference. Any particle falling within this range of sizes is termed a sand grain.

Source: Wikipedia