The Mohs scale of mineral hardness () is a

qualitative Qualitative descriptions or distinctions are based on some quality or characteristic rather than on some quantity or measured value. Qualitative may also refer to: *Qualitative property, a property that can be observed but not measured numericall ...

ordinal scale Ordinal data is a categorical, statistical data type where the variables have natural, ordered categories and the distances between the categories are not known. These data exist on an ordinal scale, one of four levels of measurement described ...

, from 1 to 10, characterizing scratch resistance of various

mineral In geology and mineralogy, a mineral or mineral species is, broadly speaking, a solid chemical compound with a fairly well-defined chemical composition and a specific crystal structure that occurs naturally in pure form.John P. Rafferty, ed. ...

s through the ability of harder material to scratch softer material. The scale was introduced in 1812 by the German

geologist A geologist is a scientist who studies the solid, liquid, and gaseous matter that constitutes Earth and other terrestrial planets, as well as the processes that shape them. Geologists usually study geology, earth science, or geophysics, alt ...

and mineralogist Friedrich Mohs, in his book ''"Versuch einer Elementar-Methode zur naturhistorischen Bestimmung und Erkennung der Fossilien"''; it is one of several definitions of

hardness In materials science, hardness (antonym: softness) is a measure of the resistance to localized plastic deformation induced by either mechanical indentation or abrasion (mechanical), abrasion. In general, different materials differ in their hardn ...

in materials science, some of which are more quantitative. The method of comparing hardness by observing which minerals can scratch others is of great antiquity, having been mentioned by

Theophrastus Theophrastus (; grc-gre, Θεόφραστος ; c. 371c. 287 BC), a Greek philosopher and the successor to Aristotle in the Peripatetic school. He was a native of Eresos in Lesbos.Gavin Hardy and Laurence Totelin, ''Ancient Botany'', Routle ...

in his treatise ''On Stones'', , followed by

Pliny the Elder Gaius Plinius Secundus (AD 23/2479), called Pliny the Elder (), was a Roman author, naturalist and natural philosopher, and naval and army commander of the early Roman Empire, and a friend of the emperor Vespasian. He wrote the encyclopedic ...

in his ''

Naturalis Historia The ''Natural History'' ( la, Naturalis historia) is a work by Pliny the Elder. The largest single work to have survived from the Roman Empire to the modern day, the ''Natural History'' compiles information gleaned from other ancient authors. ...

'', . The Mohs scale is useful for identification of minerals in the field, but is not an accurate predictor of how well materials endure in an industrial setting – ''toughness''.

Minerals

The Mohs scale of mineral hardness is based on the ability of one natural sample of mineral to scratch another mineral visibly. The samples of matter used by Mohs are all different minerals. Minerals are chemically pure solids found in nature. Rocks are made up of one or more minerals. As the hardest known naturally occurring substance when the scale was designed,

diamond Diamond is a solid form of the element carbon with its atoms arranged in a crystal structure called diamond cubic. Another solid form of carbon known as graphite is the chemically stable form of carbon at room temperature and pressure, ...

s are at the top of the scale. The hardness of a material is measured against the scale by finding the hardest material that the given material can scratch, or the softest material that can scratch the given material. For example, if some material is scratched by apatite but not by fluorite, its hardness on the Mohs scale would be between 4 and 5. "Scratching" a material for the purposes of the Mohs scale means creating non-elastic dislocations visible to the naked eye. Frequently, materials that are lower on the Mohs scale can create microscopic, non-elastic dislocations on materials that have a higher Mohs number. While these microscopic dislocations are permanent and sometimes detrimental to the harder material's structural integrity, they are not considered "scratches" for the determination of a Mohs scale number. Each of the ten hardness values in the Mohs scale is represented by a ''reference mineral'', most of which are widespread in rocks. The Mohs scale is an

. For example,

corundum Corundum is a crystalline form of aluminium oxide () typically containing traces of iron, titanium, vanadium and chromium. It is a rock-forming mineral. It is a naturally transparent material, but can have different colors depending on the ...

(9) is twice as hard as

topaz Topaz is a silicate mineral of aluminium and fluorine with the chemical formula Al Si O( F, OH). It is used as a gemstone in jewelry and other adornments. Common topaz in its natural state is colorless, though trace element impurities can ma ...

(8), but diamond (10) is four times as hard as corundum. The table below shows the comparison with the absolute hardness measured by a sclerometer, with pictorial examples. Using some ordinary materials of known hardness can be a simple way to approximate the position of a mineral on the scale. On the Mohs scale, a streak plate (unglazed

porcelain Porcelain () is a ceramic material made by heating substances, generally including materials such as kaolinite, in a kiln to temperatures between . The strength and translucence of porcelain, relative to other types of pottery, arises main ...

) has a hardness of approximately 7."Mohs hardness"
in ''Encyclopædia Britannica Online'' Other useful comparators are: finger nail (2.5), copper coin (3.5), knife blade (5.5), steel nail (6.5), masonry drill bit (8.5).

Intermediate hardness

The hardness of some minerals is intermediate between two of the Mohs scale reference minerals. Some examples are shown in the table:

Other substances

Some solid substances, which are not minerals, have been assigned a hardness on the Mohs scale. However, if the substance is actually a mixture of other substances, hardness can be difficult to determine or may be misleading or meaningless. For example, some sources have assigned a Mohs hardness of 6 or 7 to granite, but this should be treated with caution because granite is a rock made of several minerals, each with its own Mohs hardness (e.g. topaz-rich granite contains: topaz - hardness 8, quartz - hardness 7, orthoclase feldspar - hardness 6, plagioclase feldspar - hardness 6 to 6.5, mica - hardness 2 to 4).

Use

Despite its lack of precision, the Mohs scale is relevant for field geologists, who use the scale to roughly identify minerals using scratch kits. The Mohs scale hardness of minerals can be commonly found in reference sheets. Mohs hardness is useful in

milling Milling may refer to: * Milling (minting), forming narrow ridges around the edge of a coin * Milling (grinding), breaking solid materials into smaller pieces by grinding, crushing, or cutting in a mill * Milling (machining), a process of using rota ...

. It allows assessment of which kind of mill will best reduce a given product whose hardness is known. The scale is used at electronic manufacturers for testing the resilience of flat panel display components (such as cover glass for

LCD A liquid-crystal display (LCD) is a flat-panel display or other electronically modulated optical device that uses the light-modulating properties of liquid crystals combined with polarizers. Liquid crystals do not emit light directly but in ...

s or encapsulation for

OLED An organic light-emitting diode (OLED or organic LED), also known as organic electroluminescent (organic EL) diode, is a light-emitting diode (LED) in which the emissive electroluminescent layer is a film of organic compound that emits light ...

s), as well as to evaluate the hardness of touch screens in consumer electronics.

Comparison with Vickers scale

Comparison between Mohs hardness and Vickers hardness:

Minerals

Intermediate hardness

Other substances

Use

Comparison with Vickers scale

See also

References

Further reading