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Hardness, ability of a solid substance to resist surface deformation or abrasion. Various interpretations, depending on the usage, are applied to the term. In mineralogy, hardness is defined as the resistance of the smooth surface of a mineral to scratching. A soft surface is scratched more easily than a hard surface; thus a hard mineral, such as diamond, will scratch a soft mineral, such as graphite, and the hard mineral will not be scratched by the soft. The relative hardness of minerals is determined according to the Mohs scale of hardness. In the Mohs scale, named for the German mineralogist Friedrich Mohs who devised it, ten common minerals are arranged in order of increasing hardness and are assigned numbers: 1, talc; 2, gypsum; 3, calcite; 4, fluorite; 5, apatite; 6, orthoclase (feldspar); 7, quartz; 8, topaz; 9, corundum; and 10, diamond. The hardness of a mineral specimen is obtained by determining which mineral in the Mohs scale will scratch the specimen. Thus, galena, which has a hardness of 2.5, can scratch gypsum and can be scratched by calcite. The hardness of a mineral largely determines its durability.

In metallurgy and engineering, hardness is determined by impressing a small ball or cone of a hard material on the surface to be tested and measuring the size of the indentation. Hard metals are indented less than soft metals. This test to determine the hardness of metal surfaces is known as the Brinell test, named after the Swedish engineer Johann Brinell, who invented the Brinell machine for measuring the hardness of metals and alloys.

Hardness is related to the strength, durability, and toughness of solid substances, and in common usage the term is often extended to include those properties.