Hydrated silica is a form of silicon dioxide, which has a variable amount of water in the formula. When dissolved in water it is usually known as silicic acid. It is found in nature, as opal (which has been mined as a gemstone for centuries), and in the cell walls of diatoms. It is also manufactured for use in toothpaste. Once dehydrated the gel is used as a desiccant known as silica gel. It is also used in various paints and varnishes and in the production of beer. Hydrated silica is a form of silicon dioxide, which has a variable amount of water in the formula. When dissolved in water it is usually known as silicic acid. It is found in nature, as opal (which has been mined as a gemstone for centuries), and in the cell walls of diatoms. It is also manufactured for use in toothpaste. Once dehydrated the gel is used as a desiccant known as silica gel. It is also used in various paints and varnishes and in the production of beer.