Mohs scale of mineral hardness

The Mohs scale of mineral hardness is based on the ability of one natural sample of matter to scratch another. The samples of matter used by Mohs are all minerals. Minerals are pure substances found in nature. Rocks are made up of one or more minerals. As the hardest known naturally occurring substance when the scale was designed, diamonds are at the top of the scale. 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 the Mohs scale would fall between 4 and 5.

The Mohs scale is a purely ordinal scale. For example, corundum is twice as hard as topaz, but diamond is almost four times as hard as corundum. The table below shows comparison with absolute hardness measured by a sclerometer, with pictorial examples.

Since the invention of the scale, there have been reports of materials harder than the highest mineral on the scale, diamond; so the Mohs scale may be changed in the future.

Mohs hardness Mineral Absolute Hardness Image
1 Talc (Mg3Si4O10(OH)2) 1 Talc block.jpg
2 Gypsum (CaSO4·2H2O) 3 Gypsum Australia.jpg
3 Calcite (CaCO3) 9 Calcite-sample2.jpg
4 Fluorite (CaF2) 21 Fluorite with Iron Pyrite.jpg
5 Apatite (Ca5(PO4)3(OH-,Cl-,F-)) 48 Apatite crystals.jpg
6 Orthoclase Feldspar (KAlSi3O8) 72 - ortoklas.jpg
7 Quartz (SiO2) 100 Quartz Brésil.jpg
8 Topaz (Al2SiO4(OH-,F-)2) 200 Topaz cut.jpg
9 Corundum (Al2O3) 400 Cut Ruby.jpg
10 Diamond (C) 1600 Rough diamond.jpg

On the Mohs scale, a pencil "lead" (graphite) has a hardness of 1; a fingernail, 2.2-2.5; a copper penny, 3.2-3.5; a pocketknife 5.1; a knife blade, 5.5; window glass plate, 5.5; and a steel file, 6.5. A streak plate (unglazed porcelain) has a hardness of 7.0. Using these ordinary materials of known hardness can be a simple way to approximate the position of a mineral on the scale.

Aggregated diamond nanorod

Aggregated diamond nanorods, or ADNRs (also called a hyperdiamond), are a nanocrystalline form of diamond believed to be the hardest and least compressible known material. ADNRs are 0.3% denser than regular diamond. The ADNR material is also harder than type IIa diamond and ultrahard fullerite.