Hardness is the resistance of a substance to scratching, or to indentation under a blow or steady load. Resistance to scratching is measured on the Mohs' scale, named for Friedrich Mohs (1773–1839), who chose 10 minerals as reference points, from talc (hardness 1) to diamond (10). The modified Mohs' scale is now usually used, with 5 further mineral reference points.

Resistance to indentation is measured by, amongst others, the Brinell, Rockwell, and Vickers scales.