Aggregated diamond nanorods, or ADNRs, are an allotrope of carbon believed to be the hardest and least compressible known material, as measured by its isothermal bulk modulus; aggregated diamond nanorods have a modulus of 491 gigapascals (GPa), while a conventional diamond has a modulus of 442 GPa. ADNRs are also 0.3% denser than regular diamond. The ADNR material is also harder than type IIa diamond and ultrahard fullerite.

A process to produce the substance was discovered by physicists in Germany, led by Natalia Dubrovinskaia, at the University of Bayreuth in 2005. ADNRs are made by compressing allotropic carbon fullerene molecules (generally 60 carbon atoms per molecule) to a pressure of 20 GPa, while at the same time heating to 2500 kelvin, using a unique 5000-metric-ton multi anvil press. The resulting substance is a series of interconnected diamond nanorods, with diameters of between 5 and 20 nanometres and lengths of around 1 micrometre each.

Artistic rendition of a diamond anvil cell

A diamond anvil cell, located at the European Synchrotron Radiation Facility at Grenoble, France, was used to measure the compressibility of the material.

Boron carbide (chemical formula B₄C) is an extremely hard ceramic material used in tank armor, bulletproof vests, and numerous industrial applications. With a hardness of 9.3 on the mohs scale, it is the fifth hardest material known behind boron nitride, diamond, ultrahard fullerite, and aggregated diamond nanorods.

Discovered in the 19th century as a by-product of reactions involving metal borides, it was not until the 1930s that the material was studied scientifically. Boron carbide is now produced industrially by the carbo-thermal reduction of B₂O₃ (boron oxide) in an electric arc furnace.

Its ability to absorb neutrons without forming long lived radionuclides makes the material attractive as an absorbent for neutron radiation arising in nuclear power plants. Nuclear applications of boron carbide include shielding, control rod and shut down pellets. Within control rods, boron carbide is often powdered, to increase its surface area.