Boron carbide, also known as black diamond, has the molecular formula B₄C and typically appears as a gray-black micro-powder. It is one of the three hardest known materials (the other two being diamond and cubic boron nitride) and is used in tank armor, bulletproof vests, and many industrial applications. Its Mohs hardness is 9.3.

Boron carbide can absorb a large number of neutrons without forming any radioactive isotopes, making it an ideal neutron absorber in nuclear power plants. Neutron absorbers are primarily used to control the rate of nuclear fission. In nuclear reactors, boron carbide is mainly made into control rods, but sometimes it is ground into powder form to increase its surface area.

Due to its low density, high strength, excellent high-temperature stability, and good chemical stability, it is used in wear-resistant materials, ceramic reinforcement phases, and especially in lightweight armor and reactor neutron absorbers. Additionally, compared to diamond and cubic boron nitride, boron carbide is easier to manufacture, making it more widely used. In certain applications, it can replace expensive diamond, commonly seen in grinding, polishing, drilling, and other uses.