The Mohs Hardness Scale

A relative scale for measuring mineral hardness

Quartz
Quartz is the standard for Mohs hardness 7. Andrew Alden photo

Mohs hardness scale was devised in 1812 by Friedrich Mohs and has been the same ever since, making it the oldest standard scale in geology. It is also perhaps the most useful single test for identifying and describing minerals. You use the Mohs hardness scale by testing an unknown mineral against one of the standard minerals. Whichever one scratches the other is harder, and if both scratch each other they are the same hardness.

Understanding Mohs Hardness Scale

The Mohs scale of hardness uses half-numbers, but nothing more precise for in-between hardnesses. For instance, dolomite, which scratches calcite but not fluorite, has a Mohs hardness of 3½ or 3.5. 

Mohs HardnessMineral NameChemical Formula
1TalcMg3Si4O10(OH)2
2GypsumCaSO4·2H2O
3CalciteCaCO3
4FluoriteCaF2
5ApatiteCa5(PO4)3(F,Cl,OH)
6FeldsparKAlSi3O8 – NaAlSi3O8 – CaAl2Si2O8
7QuartzSiO2
8TopazAl2SiO4(F,OH)2
9CorundumAl2O3
10DiamondC

 

There are a few handy objects that also help in using this scale. A fingernail is 2½, a penny (actually, any current U.S. coin) is just under 3, a knife blade is 5½, glass is 5½ and a good steel file is 6½. Common sandpaper uses artificial corundum and is hardness 9; garnet paper is 7½.

Many geologists just use a small kit featuring 9 standard minerals and some of the above-mentioned objects; with the exception of diamond, all of the minerals on the scale are fairly common and inexpensive.

 If you want to avoid the rare chance of a mineral impurity skewing your results (and don't mind spending some extra money), there are sets of hardness picks available specifically for the Mohs scale. 

The Mohs scale is an ordinal scale, meaning that it is not proportional. In terms of absolute hardness, diamond (Mohs hardness 10) is actually four times harder than corundum (Mohs hardness 9) and six times harder than topaz (Mohs hardness 8).

 For a field geologist, the scale works great. A professional mineralogist or metallurgist, however, might obtain absolute hardness by using a sclerometer, which microscopically measures the width of a scratch made by a diamond. 

Mineral NameMohs Hardness Absolute Hardness
Talc11
Gypsum22
Calcite39
Fluorite421
Apatite548
Feldspar672
Quartz7100
Topaz8200
Corundum9400
Diamond101500

 

Mohs hardness is just one aspect of identifying minerals. You also need to consider luster, cleavage, crystalline form, color, and rock type to zero in on an exact identification. See this step-by-step guide to mineral identification to learn more.

A mineral's hardness is a reflection of its molecular structure -- the spacing of the various atoms and the strength of the chemical bonds between them. The manufacture of Gorilla Glass used in smartphones, which is nearly hardness 9, is a good example of how this aspect of chemistry is related to hardness. Hardness is also an important consideration in gemstones.

Don't rely on the Mohs scale to test rocks; it is strictly for minerals. The hardness of a rock depends on the exact minerals that make it up, particularly the mineral that cements it together.

Edited by Brooks Mitchell