IN NATURE of Jan. 23, 1926 (vol. 117, p. 117), I gave a description of a method of determining Hardness, in which a cone of the material to be tested was pressed against a hard surface (sapphire ...

Skoltech researchers have presented a new simple physical model for predicting the hardness of materials based on information about the shear modulus and equations of the state of crystal structures.

Manufacturers use them to chart materials' scratch resistance. You can even use one yourself to figure out the hardness of any given material. The hardest pencil, rated 10H, corresponds to roughly ...