Sand is a very generic term referring to a granulated mineral mixture. The more common components of sands are silicon dioxide and aluminum oxide, although substantial amounts of pulverized calcium carbonate from shells or limestone deposits are often present. In certain places sand can be composed almost entirely of gypsum.
Sand that is substantially pure silicon dioxide is called "silica sand" and can be used as a starting material for glass. Very fine sands composed of mixtures of silicon and aluminum oxides form clays like kaolinite.
Tiny amounts of silicic acid H2SiO3, form at the surface of silicon-bearing minerals as a result of the hydration of silicon dioxide. Finely divided silicon dioxide, like volcanic ash, has so very much surface area that this sort of reaction happens at an appreciable rate. Sand that contains a sufficient amount of silicic acid is called "pozzolan,".