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Mohs Scale of Mineral Hardness

Mohs scale of mineral hardness characterises 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 mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science.

Hardness Mineral Absolute Hardness
0.4 Potassium (K)  
0.5 Sodium (Na)  
1 Talc (Mg3Si4O10(OH)2) 1
1.5 Lead (Pb), Tin (Sn)  
2 Gypsum (CaSO4•2H2O) 2
2.5 Fingernail, gold (Au), magnesium (Mg), silver (Ag)  
2.75 Aluminium (Al)  
3 Calcite (CaCO3) 9
3 Copper (Cu)  
3.5 Brass  
4 Fluorite (CaF2) 21
4 Iron (Fe)  
5 Apatite (Ca5(PO4)3(OH-,Cl-,F-)) 48
5 Steel  
6 Orthoclase Feldspar (KAlSi3O8) 72
6 Glass  
7 Quartz (SiO2) 100
7 Hardened steel (eg, a file)  
7.5-8.0 Beryl (Be3Al2(SiO3)6 – eg, emerald)  
8 Topaz (Al2SiO4(OH-,F-)2) 200
8 Emery (impure form of Al2O3)  
9 Corundum (Al2O3) 400
10 Diamond (C) 1,500
  Aggregated diamond nanorods (C) 1,666*

Mohs based the scale on ten minerals (shown in bold with a grey background) that are all readily available except the last one, diamond. 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 Mohs scale is 4.5.

The table shows comparison with absolute hardness measures from a sclerometer. Mohs' is a purely ordinal scale with, for example, corundum being twice as hard as topaz, but diamond almost four times as hard as corundum. A sclerometer (from the Ancient Greek skleros meaning "hard") is a mineralogist's instrument used to measure the hardness of materials. The instrument is designed to determine the degree of hardness of a given mineral by applying pressure on a moving diamond point until a "scratch" has occurred.

Notes

Because glass is harder than steel, a sharp-edged kitchen knife should never be used on a glass cutting surface. It will dull the blade.

* Aggregated diamond nanorods (ADNR) are 11% less compressible than diamond in a diamond anvil cell. Testing the Vickers microhardness (using a diamond indenter) showed directly that the probe tip failed to make an indentation on the surface of the ADNR and ADNR can scratch faces of type-IIa natural diamonds, thus ADNR is harder than natural diamond and consequently more resistant against abrasion. The random arrangement of the nanorods most probably gives rise to the increased hardness of ADNR.