Boron nitride is a binary chemical compound, consisting of equal proportions of boron and nitrogen, with formula BN. Structurally, it is isoelectronic to carbon and takes on similar physical forms: a graphite-like one, and a diamond-like one. The latter is one of the hardest materials known, behind diamond and ultrahard fullerite. It is widely used for grinding and as material for tools in industry, partly because it does not dissolve into iron, nickel and related alloys at high temperatures, but diamond does. Theoretical beta carbon nitride is thought to be harder. Hexagonal boron nitride finds use as a high-temperature lubricant where the electrical conductivity or reactivity of graphite would be problematic.

It can also be synthesized in forms analogous to carbon nanotubes, but with a more uniform electronic band gap and the potential for piezoelectric activity.