The Mohs Scale of Mineral Hardness was developed in 1812 by a German mineralogist, Frederich Mohs (his name rhymes with “toes”). His simple, ten-step scale has become a basic testing tool widely used by geologists and geology students for identifying mineral samples.
The Mohs Scale is a relative scale because, instead of using an instrument to make a measurement, an unknown mineral’s hardness (or resistance to scratching) is compared to the hardness of the minerals that make up the scale. Since by definition every mineral has specific composition and physical properties, a mineral’s property of hardness will fall within a narrow range, and can thus be used to help differentiate it from other minerals.
Mohs chose minerals for his scale that are widespread and (except for diamond) readily available. From softest to hardest, the ten minerals are:
1: talc - hydrated magnesium silicate –- Mg3Si4O10(OH)2
2: gypsum - hydrated calcium sulfate -– CaSO4·2H2O
3: calcite - calcium carbonate -– CaCO3
4: fluorite - calcium fluorite –- CaF2
5: apatite - hydrated calcium phosphate –- Ca5(PO4)3(OH-, Cl, F-)
6: orthoclase feldspar - potassium-aluminum silicate –- KalSi3O8
7: quartz - silicon dioxide -– SiO2 (the most abundant mineral on earth)
8: topaz - hydrated aluminum silicate -– AlSiO4(OH-, F-)2
9: corundum - aluminum oxide -– Al2O3
10: diamond - carbon - C
The Mohs Scale is used in a comparative process: if one of the standard minerals can scratch the surface of the unknown mineral, the standard is harder. If the unknown mineral scratches the standard mineral, then the unknown is harder. For instance, a mineral that can scratch calcite but is scratched by fluorite has a hardness between three and four. Mineral identification charts or keys list hardness or hardness range as one of the identifying characteristics of each mineral.
While a variety of kits containing samples of each mineral can be obtained through scientific suppliers, a simplified version of the Mohs scale has also been formulated that uses common items. In this version of the scale, a human fingernail has a hardness of about 2½, a copper penny is about three, pocket knife blades are about 5½, and ordinary window glass is between six and seven.
The Mohs Scale is relative, and its steps are based on arbitrarily-chosen mineral samples, so the steps between the defining minerals are not even. For instance, diamond (10) is four times as hard as corundum (9), and corundum is twice as hard as topaz (8). A mechanical device known as a sclerometer is used to determine the absolute hardness of a mineral sample. Based on measurements using this instrument, diamond has been determined to be 1600 times as hard as talc and sixteen times as hard as quartz.