The Mohs scale of mineral hardness reference article from the English Wikipedia on 24-Jul-2004
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Mohs scale of mineral hardness

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Mohs' scale of mineral hardness characterizes the scratch resistance of various minerals through the ability of a harder material to scratch a softer. It was created by the German mineralogist Friedrich Mohs and is one of several definitions of hardness in materials science.

Mohs based the scale on ten readily available minerals. Materials are characterised against the scale by finding the hardest material that they can scratch.

The table below shows comparison with absolute hardness measures by 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.

Hardness Mineral Absolute Hardness
1 Talc (Mg3Si4O10(OH)2) 1
2 Gypsum (CaSO4·2H2O) 3
3 Calcite (CaCO3) 9
4 Fluorite (CaF2) 21
5 Apatite (Ca5(PO4)3(OH-,Cl-,F-)) 48
6 Orthoclase Feldspar (KAlSi3O8) 72
7 Quartz (SiO2) 100
8 Topaz (Al2SiO4(OH-,F-)2) 200
9 Corundum (Al2O3) 400
10 Diamond (C) 1500

The mnemonic traditionally taught to geology students to remember this table is "The Girls Can Flirt And Other Queer Things Can Do."

An alternative table is shown below which has been modified to incorporate additional substances that may fall in between two levels.

Hardness Mineral
1 Liquid
2 Gypsum
3 Calcite
4 Fluorite
5 Apatite
6 Orthoclase
7 Vitreous pure silica
8 Quartz
9 Topaz
10 Garnet
11 Fused zirconia
12 Fused alumina
13 Silicon carbide
14 Boron carbide
15 Diamond