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?
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 HARDNESS | MINERAL | CHEMICAL FORMULA | OBSERVATIONS |
|---|---|---|---|
| 1 | talc | Mg3Si4O10(OH)2 | can be easily scratched by the fingernail |
| 2 | gypsum | CaSO4·2H2O | can be scratched by the fingernail |
| 3 | calcite | CaCO3 | can be easily scratched with a knife |
| 4 | fluorite | CaF2 | can be easily scratched with a knife but not as easily as calcite |
| 5 | apatite | Ca5(PO4)3(OH−,Cl−,F−) | can be scratched with a knife with difficulty |
| 6 | orthoclase | KAlSi3O8 | cannot be scratched with a knife but scratches glass with difficulty |
| 7 | quartz | SiO2 | scratches glass easily |
| 8 | topaz | Al2SiO4(OH−,F−)2 | scratches glass very easily |
| 9 | corundum | Al2O3 | cuts glass |
| 10 | diamond | C | used as a glass cutter |
Mohs Hardness Test
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 a 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
| MINERAL | MOHS HARDNESS |
|---|---|
| anhydrite | 3 to 3.5 |
| apatite | 5 |
| arsenopyrite | 5.5 to 6 |
| augite | 5.5 to 6 |
| azurite | 3.5 to 4 |
| barite | 2.5 to 3.5 |
| bauxite | 1 to 3 |
| beryl | 7.5 to 8 |
| biotite | 2.5 to 3 |
| bornite | 3 to 3.25 |
| calcite | 3 |
| cassiterite | 6 to 7 |
| chalcocite | 2.5 to 3 |
| chalcopyrite | 3.5 to 4 |
| chlorite | 2 to 2.5 |
| chromite | 5.5 to 6 |
| chrysoberyl | 8.5 |
| cinnabar | 2 to 2.5 |
| copper | 2.5 to 3 |
| cordierite | 7 to 7.5 |
| corundum | 9 |
| cuprite | 3.5 to 4 |
| diamond | 10 |
| diopside | 5.5 to 6.5 |
| dolomite | 3.5 to 4 |
| enstatite | 5 to 6 |
| epidote | 6 to 7 |
| euclase | 7.5 |
| fluorite | 4 |
| galena | 2.5+ |
| garnet | 6.5 to 7.5 |
| glauconite | 2 |
| gold | 2.5 to 3 |
| graphite | 1 to 2 |
| gypsum | 1.5 to 2 |
| halite | 2 to 2.5 |
| hematite | 5 to 6.5 |
| hornblende | 5 to 6 |
| ilmenite | 5 to 6 |
| jadeite | 6.5 to 7 |
| kyanite | 4.5 to 7 |
| limonite | 1 to 5 |
| magnesite | 3.5 to 5 |
| magnetite | 5 to 6.5 |
| malachite | 3.5 to 4 |
| marcasite | 6 to 7.5 |
| molybdenite | 1 to 2 |
| monazite | 5 to 5.5 |
| muscovite | 2 to 3 |
| nepheline | 5.5 to 6 |
| nephrite | 6 to 6.5 |
| olivine | 6.5 to 7 |
| orthoclase | 6 to 6.5 |
| plagioclase | 6 to 6.5 |
| platinum | 3.5 |
| prehnite | 6 to 6.5 |
| pyrite | 6 to 6.5 |
| pyrophyllite | 1 to 2 |
| pyrrhotite | 3.5 to 4 |
| quartz | 7 |
| rhodochrosite | 3.5 to 4 |
| rhodonite | 5.5 to 6.5 |
| rutile | 6 to 6.5 |
| serpentine | 3 to 5 |
| siderite | 3.5 to 4.5 |
| sillimanite | 6.5 to 7.5 |
| silver | 2.5 to 3 |
| sodalite | 5.5 to 6 |
| sphalerite | 3.5 to 4 |
| spinel | 8 |
| spodumene | 6.5 to 7 |
| staurolite | 7 to 7.5 |
| sulfur | 1.5 to 2.5 |
| sylvite | 2 |
| talc | 1 |
| titanite | 5 to 5.5 |
| topaz | 8 |
| tourmaline | 7 to 7.5 |
| turquoise | 5 to 6 |
| uraninite | 5 to 6 |
| vanadinite | 3 to 4 |
| witherite | 3 to 3.5 |
| wollastonite | 4.5 to 5.5 |
| zircon | 7.5 |
| zoisite | 6 to 7 |
Featured image: Miriam Doerr Martin Frommherz / Shutterstock
