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Mohs Scale Of Hardness
Gemstones

Mohs Scale of Hardness

Last updated on December 27, 2022

Hardness is one of the main characteristics of a gemstone. It defines how resistant is the gem to scratches and may be described relative to a standard hardness scale widely known as the Mohs scale of hardness.

What Is the Mohs Scale of Hardness?

Mohs Scale Of Hardness | Diamond Buzz
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The Mohs scale of hardness (pronounced “moze”) is a 1-to-10 qualitative scale characterising scratch and abrasion resistance of minerals (solid, naturally occurring inorganic substances) through the ability of harder material to scratch softer material. The scale was introduced by German geologist and mineralogist Friedrich Mohs in his Treatise on Mineralogy: Or, The Natural History of the Mineral Kingdom in 1822.

It is worth mentioning that the method of comparing hardness by observing which mineral can scratch others has been described by Theophrastus in his treatise On Stones, circa 300 BC. However, this method was not widely used until Friedrich Mohs selected ten minerals, representing each degree of hardness and assigned numbers to them considering the relative ease or difficulty with which one mineral can be scratched by another.

According to the Mohs scale, the hardness of talc is 1, gypsum is 2, calcite is 3, fluorite is 4, apatite is 5, orthoclase is 6, quartz is 7, topaz is 8, corundum is 9, and diamond is at the top of the scale with the hardness of 10.

Mohs Scale of Hardness

MOHS HARDNESSMINERALCHEMICAL FORMULAOBSERVATIONS
1talcMg3Si4O10(OH)2can be easily scratched by the fingernail
2gypsumCaSO4·2H2Ocan be scratched by the fingernail
3calciteCaCO3can be easily scratched with a knife
4fluoriteCaF2can be easily scratched with a knife but not as easily as calcite
5apatiteCa5(PO4)3(OH−,Cl−,F−)can be scratched with a knife with difficulty
6orthoclaseKAlSi3O8cannot be scratched with a knife but scratches glass with difficulty
7quartzSiO2scratches glass easily
8topazAl2SiO4(OH−,F−)2scratches glass very easily
9corundumAl2O3cuts glass
10diamondCused as a glass cutter

Mohs Hardness Test

Mohs Hardness Kit | Diamond Buzz
Image: Hannes Grobe, CC BY 3.0 / Wikimedia Commons. A derivative work by Diamond Buzz.

Determining the hardness of an unknown mineral is a part of its identification process. To measure the hardness of a mineral against the Mohs hardness scale, we need to find the hardest material that the given mineral can scratch or the softest material that can scratch the given mineral. For example, if the given mineral is scratched by orthoclase but not by apatite, its hardness on the Mohs scale would be between 5 and 6.

The Mohs hardness test is conducted by placing a sharp point of one mineral on the smooth surface of another mineral and attempting to produce a scratch. In reference to the Mohs scale, “scratch” means creating non-elastic dislocations visible to the naked eye. Often, materials can create microscopic, non-elastic dislocations on harder materials. However, these dislocations are not considered “scratches” even though they are permanent and can affect the harder material’s structural integrity.

It is noteworthy that the Mohs scale of mineral hardness does not indicate exact hardness, which depends on the purity of the sample and its degree of crystallinity. Also, many minerals have variable hardness depending on the direction in which they are being scratched. An example of a mineral with variable hardness is kyanite.

It is also worth mentioning that the Mohs hardness kit, containing one specimen of each mineral on the ten-point hardness scale, can be replaced with a set of hardness picks. These tools have sharp metal points giving very accurate results and can be used to test small mineral grains in a rock, allowing much more control.

Mohs Hardness of Common Minerals

MINERALMOHS HARDNESS
anhydrite3 to 3.5
apatite5
arsenopyrite5.5 to 6
augite5.5 to 6
azurite3.5 to 4
barite2.5 to 3.5
bauxite1 to 3
beryl7.5 to 8
biotite2.5 to 3
bornite3 to 3.25
calcite3
cassiterite6 to 7
chalcocite2.5 to 3
chalcopyrite3.5 to 4
chlorite2 to 2.5
chromite5.5 to 6
chrysoberyl8.5
cinnabar2 to 2.5
copper2.5 to 3
cordierite7 to 7.5
corundum9
cuprite3.5 to 4
diamond10
diopside5.5 to 6.5
dolomite3.5 to 4
enstatite5 to 6
epidote6 to 7
euclase7.5
fluorite4
galena2.5+
garnet6.5 to 7.5
glauconite2
gold2.5 to 3
graphite1 to 2
gypsum1.5 to 2
halite2 to 2.5
hematite5 to 6.5
hornblende5 to 6
ilmenite5 to 6
jadeite6.5 to 7
kyanite4.5 to 7
limonite1 to 5
magnesite3.5 to 5
magnetite5 to 6.5
malachite3.5 to 4
marcasite6 to 7.5
molybdenite1 to 2
monazite5 to 5.5
muscovite2 to 3
nepheline5.5 to 6
nephrite6 to 6.5
olivine6.5 to 7
orthoclase6 to 6.5
plagioclase6 to 6.5
platinum3.5
prehnite6 to 6.5
pyrite6 to 6.5
pyrophyllite1 to 2
pyrrhotite3.5 to 4
quartz7
rhodochrosite3.5 to 4
rhodonite5.5 to 6.5
rutile6 to 6.5
serpentine3 to 5
siderite3.5 to 4.5
sillimanite6.5 to 7.5
silver2.5 to 3
sodalite5.5 to 6
sphalerite3.5 to 4
spinel8
spodumene6.5 to 7
staurolite7 to 7.5
sulfur1.5 to 2.5
sylvite2
talc1
titanite5 to 5.5
topaz8
tourmaline7 to 7.5
turquoise5 to 6
uraninite5 to 6
vanadinite3 to 4
witherite3 to 3.5
wollastonite4.5 to 5.5
zircon7.5
zoisite6 to 7

Featured image: Miriam Doerr Martin Frommherz / Shutterstock
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