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The Mohs scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer material. It was created in 1812 by the German geologist and mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science. The method of comparing hardness by seeing which minerals can scratch others, however, is of great antiquity, having been mentioned by Theophrastus in his treatise On Stones, c. 300 BC, followed by Pliny the Elder in his Naturalis Historia, c. 77 AD.
The Mohs scale of mineral hardness is based on the ability of one natural sample of matter to scratch another mineral. The samples of matter used by Mohs are all different minerals. Minerals are pure substances found in nature. Rocks are made up of one or more minerals. As the hardest known naturally occurring substance when the scale was designed, diamonds 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, and/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 fall between 4 and 5.
The Mohs scale is a purely ordinal scale. For example, corundum (9) is twice as hard as topaz (8), but diamond (10) is four times as hard as corundum. The table below shows comparison with absolute hardness measured by a sclerometer, with pictorial examples.
|Mohs hardness||Mineral||Chemical formula||Absolute hardness||Image|
On the Mohs scale, graphite (a principal constituent of pencil "lead") has a hardness of 1.5; a fingernail, 2.2–2.5; a copper penny, 3.2–3.5; a pocketknife 5.1; a knife blade, 5.5[clarification needed]; window glass plate, 5.5; and a steel nail, 5.5. A streak plate (unglazed porcelain) has a hardness of 7.0. Using these ordinary materials of known hardness can be a simple way to approximate the position of a mineral on the scale.
The table below incorporates additional substances that may fall between levels:
|Hardness||Substance or mineral|
|0.5–0.6||lithium, sodium, potassium|
|1.5||gallium, strontium, indium, tin, barium, thallium, lead, graphite, ice|
|2||hexagonal boron nitride, calcium, selenium, cadmium, sulfur, tellurium, bismuth|
|2.5–3||magnesium, gold, silver, aluminium, zinc, lanthanum, cerium, Jet (lignite)|
|3||calcite, copper, arsenic, antimony, thorium, dentin|
|4||fluorite, iron, nickel|
|5||apatite (tooth enamel), cobalt, zirconium, palladium, obsidian (volcanic glass)|
|5.5||beryllium, molybdenum, hafnium|
|6||orthoclase, titanium, manganese, germanium, niobium, rhodium, uranium|
|6–7||glass, fused quartz, iron pyrite, silicon, ruthenium, iridium, tantalum, opal, peridot, tanzanite|
|7||osmium, quartz, rhenium, vanadium|
|7.5–8||emerald, hardened steel, tungsten, spinel|
|8||topaz, cubic zirconia|
|8.5||chrysoberyl, chromium, silicon nitride, tantalum carbide|
|9–9.5||corundum, silicon carbide (carborundum), tungsten carbide, titanium carbide|
|9.5–10||boron, boron nitride, rhenium diboride, stishovite, titanium diboride|
|>10||nanocrystalline diamond (hyperdiamond, ultrahard fullerite)|
|Hardness (Mohs)||Hardness (Vickers)|