Mohs hardness scale
The Mohs scale ranks minerals by scratch resistance: each mineral scratches those below it and is scratched by those above. It is ordinal, not linear — diamond is far harder than corundum than the one-step gap suggests.
| Hardness | Mineral | Chemical formula | Scratched by |
|---|---|---|---|
| 1 | Talc | Mg3Si4O10(OH)2 | Fingernail |
| 2 | Gypsum | CaSO4·2H2O | Fingernail (2.5) |
| 3 | Calcite | CaCO3 | Copper coin (3.5) |
| 4 | Fluorite | CaF2 | Steel knife (5.5) |
| 5 | Apatite | Ca5(PO4)3(F,Cl,OH) | Steel knife (5.5) |
| 6 | Orthoclase feldspar | KAlSi3O8 | Steel file (6.5) |
| 7 | Quartz | SiO2 | Topaz and above |
| 8 | Topaz | Al2SiO4(F,OH)2 | Corundum and above |
| 9 | Corundum | Al2O3 | Diamond only |
| 10 | Diamond | C | Nothing in the scale |