Final answer:
In hardness tests, a diamond cone or sphere is pressed into a material's surface to measure its resistance to deformation, indicated by the indentation left. This resistance is because of diamond's strong covalent bonding and three-dimensional lattice structure.
Step-by-step explanation:
In hardness tests, such as the Rockwell, Vickers, and Brinell hardness tests, a diamond cone or sphere is forced into the surface of the material being tested. The degree of hardness is determined by the depth or size of the indentation left by the diamond, which is a reflection of the material's resistance to deformation. This resistance is due to the strong covalent bonds in diamond's structure, where each carbon atom forms four single bonds to four other atoms, creating a three-dimensional lattice that extends throughout the crystal, making it extremely hard and with a high melting point.