First, let us define what sand is. Your first thought may be, “Why? I already know what sand is!” What you may not know is that in geology, sand is strictly defined as sedimentary particles between 0.0625 (1/16) mm and 2mm in diameter. Coarser particles are calledgranule, while finer particles are known as silt.

Most sediments, including sand, are made up of the fragments that result when rock is broken down by wind and rain (weathering). Generally, they start as larger fragments (gravel), which are broken down as rivers carry them down stream; the finer the particle, the further it has traveled. In other words, large bits of gravel are plentiful on the banks close to the head of a river. As it travels down stream, gravel becomes finer into cobble, pebble, granule, and eventually turning into sand.



In most cases, each grain of sand is made up of a single crystal of a mineral. Rocks are amalgamations of numerous minerals, but in the course of their being broken down and transported, they separate into minerals; by the time the fragments reach the size of sand, each particle consists of a single mineral. Composite particles, which have not broken down to single crystals, do also exist, especially in relatively coarse sand. These are known as lithic fragments. When non-crystalline rocks (such as mudstone and chert) are broken down, lithic fragments, rather than minerals, are formed.
Obsidian (volcanic glass) is not a mineral, but it also can be sand when it is broken down. On the seashore, fossilized carcasses of planktons with carbonate or silicate exoskeletons may also accumulate as sand. A famous example is star sand, which can be found on certain islands in the south of Japan.