The primary difference between sand and silt is the particle size. Sand is composed of large particles, making it excessively coarse. Silt is made of much smaller particles and is slippery to the touch.
Sand and silt are both examples of granular materials, which are collections of separate particles and distinct particles, such as raw flour in a container or jellybeans in a jar.
The most common forms of sand and silt are composed of tiny fragments of quartz. Quartz is a mineral that forms from volcanic activity deep beneath the crust of the Earth. Through erosion and weathering, quartz crystals are slowly broken up over millions of years. These fragmented crystals are deposited by rivers and oceans onto beaches and riverbanks, which becomes the sandy shore. Quartz crystals that are weathered even further sink to the bottom of riverbeds and the ocean floor to form a silt lining.
Other granular materials include gravel, which is composed of particles that are larger and coarser than sand, and clay, which is made up of particles finer than silt.
Geologists precisely measure the diameter of particles to determine their group. They use standards, such as the Udden-Wentworth scale, to determine the category for a collection of particles.