The chemical formula for most glass is SiO2. Commonly referred to as sand, it's the main ingredient in the most common forms of glass.
In it's natural state, silicone dioxide (sand) has a crystalline structure. To make glass, it is generally mixed with Na2CO3 (sodium carbonate) and either CaCo3 (calcium carbonate or lime) or MgCO3 (dolomite). The silicone dioxide is then heated at extremely high temperatures to create a molten state, then cooled extremely rapidly to create the glass. Depending on the type of oxide-based glass, the ingredients may vary somewhat. Pure-oxide glass, used for laboratory glassware and fiber optics, consists of only sand.