Glass is a hard material that can be made in many shapes. It is usually transparent, but it can also be made in colours. Glass is mainly made of silica; glass made of silica only is called silica glass. It originated in India.

Glass used to make windows and bottles is a specific type called soda-lime glass, composed of about 75% silicon dioxide (SiO2), sodium oxide (Na2O) from sodium carbonate (Na2CO3), calcium oxide, also called lime (CaO), and several minor additives.

By changing the proportions, and adding different ingredients, many kinds of glass can be made. Coloured glass is made by adding small amounts of metal oxides. For example, a blue colour is given by tiny amounts of cobalt oxide.