Mohs' hardness scale provides an index and relative measure of mineral hardness (i.e., resistance to abrasion).
In 1812, German geologist Frierich Mohs (1773–1839) devised a scale with specimen minerals that offered comparison of hardness qualities that allows the assignment of a Mohs hardness number to a mineral. Mohs' scale utilizes ten specific representative materials that are arranged numerically from the softest (1) to the hardest (10). The reference minerals are (1) talc, (2) gypsum, (3) calcite, (4) fluorite, (5) apatite, (6) orthoclase feldspar, (7) quartz, (8) topaz, (9) corundum, and (10) diamond.
The softest mineral, talc, can be used in body powder. The hardest, diamond, is used in drill bits to cut through the most dense crustal materials. Mohs' scale is a relative index scale, meaning that a determination of Mohs' hardness number for a mineral is based upon scratch tests. For example, gypsum (Mohs' hardness number = 2) will scratch talc (Mohs' hardness number = 1). Talc, however, will not scratch gypsum. Glass is assigned a Mohs hardness number of 5.5 because it will scratch apatite (Mohs' hardness number = 5) but will not scratch orthoclase feldspar (Mohs' hardness number = 6).
Scratch tests are a common method used to identify mineral hardness relative to Mohs' scale. Streak tests are often carried out on streak plates. Mineral hardness is a fundamental property of minerals and can be used to identify unknown minerals. In the absence of comparative minerals, geologists often resort to common objects with a relatively well-established Mohs' hardness number. In addition to glass (5.5), copper pennies measure 3.5, and the average human fingernail averages a Mohs' hardness of 2.5.
The Mohs' scale is a comparative index rather than a linear scale. In fact, Mohs' scale has a near logarithmic relationship to absolute hardness. At the lower, softer end of the scale, the difference in hardness is close to linear, but at the extremes of hardness, there are much greater increases in absolute hardness (e.g., a greater increase in the hardness between corundum and diamond than between quartz and topaz).
Hardness is a property of minerals derived from the nature and strength of chemical bonds in and between crystals. The number of atoms and the spatial density of bonds also influences mineral hardness. Softer minerals are held together by weak van der Waals bonds. The hardest minerals tend to be composed of dense arrays of atoms covalently bonded together.
Hardness characteristics—especially in calcite crystals—may vary as a property dependent upon the direction of the scratch (i.e., able show evidence of a particular Mohs' number if scratched along one face or direction as opposed to a different hardness number if scratched in a different direction.