Diamond has a Mohs hardness of 10 and a microhardness of 98.5 GPa. The theory of hardness was largely a semi-empirical science, and even until recently, researchers have attempted to relate hardness to the structure of chemical bonds. Some relationships have been established between a material's wear resistance and lattice energy, as well as between atomic interstices and hardness. The results obtained by different researchers are quite similar, with most tending to attribute hardness to atomic number density and bond energy density.
Of all known substances, diamond is the hardest and has the highest atomic number density. Based on their hardness, a group of superhard materials can be identified, including diamond, diamond-like solids, cubic boron nitride, wurtzite hexagonal boron nitride, silicon carbide, and boron carbide. On average, the microhardness of superhard materials is 2 to 3 orders of magnitude higher than that of hardened steel.
Diamond conducts heat via phonons, resulting in excellent thermal conductivity. Diamond has the highest thermal conductivity of any known substance, and its thermal conductivity is highly stable at room temperature. Specially cut diamond material can be used as window material for large equipment and high-heat-generating devices, providing heat dissipation.
Solid-state physics shows that an infinite number of parallel planes can be formed within the spatial lattice of a crystal. Because the spacing between different families of crystal planes varies, under certain external forces, the crystal will split along the plane with the largest spacing. This phenomenon is called dissociation. Diamond crystals have family planes with the largest spacing, making them most susceptible to cleavage along these planes.