Beta carbon nitride (β-C3N4) is a theoretical material, derived from theories on crystalline structure. It has not been made yet. It is expected to be harder than diamond but not as hard as Ultrahard fullerite.
Ultrahard fullerite (C60) is a form of carbon found to be harder than diamond, and which can be used to create even harder materials. A Type IIa diamond (111) has a hardness value of 167±6 gigapascals (GPa) when scratched with an ultrahard fullerite tip. A Type IIa diamond (111) has a hardness value of 231±5 GPa when scratched with a diamond tip; this leads to hypothetically inflated values.
Ultrahard fullerite has a hardness value of 310 GPa, though the actual value may range ±40 GPa, since testing done using an ultrahard fullerite tip on ultrahard fullerite will lead to, like diamond on diamond, distorted values.
Ultrahard fullerite can be made into Aggregated diamond nanorods. 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. ADNRs are made by compressing allotropic Carbon buckyballs molecules (generally 60 Carbon atoms per molecule) to a pressure of 20 GPa, while at the same time heating to 2500 kelvins, using a unique 5000 metric tonne 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.