MOHS hardness scale

Invented in 1812 by the German mineralogist Friedrich Mohs 

The Mohs scale measures the hardness of minerals by a comparison process.

Hardness  Mineral Chemical composition
1 Talc, crumbly under the nail Mg3Si4O10 (OH) 2
2 Gypsum, scratchable with the nail CaSO4 · 2H2O
3 Calcite, scratchable with a copper piece CaCO3
4 Fluorite, scratchable (easily) with a knife CaF2
5 Apatite, scratchable with a knife Ca5 (PO4) 3 (OH-, Cl-, F-)
6 Orthosis, scratchable with the file, by the sand KAlSi3O8
7 Quartz, scratch glass SiO2
8 Topaz, scratched by tungsten carbide Al2SiO4 (OH-, F-) 2
9 Corundum, scratchable with silicon carbide Al2O3
10 Diamond, scratchable with another diamond VS