Sample of the element boron B rotating on a watch glass. Boron has the atomic number five, and it is a rare metalloid element that exists either as a very hard black crystalline form, or an amorphous brown form. Major industrial_scale uses of boron compounds are in sodium perborate bleaches, and the borax component of fibreglass insulation. Hydrated sodium borate borax is the main source of boric acid, which is used as a cleaning product. It is also used as a dopant in semiconductors.