Moh's Scale of Hardness
The universal scale used to test the hardness of a mineral or rock is known as Moh's scale. It was devised by the Austrian mineralogist Frederick Moh in the early 1800's as a crude but practical method of comparing scratch resistance. Hardness can be tested by comparing resistance to fracture, indentation or breakage however Moh's scale refers only to resistance to abrasion.
| Hardness | Mineral | Occurances & Uses |
|---|---|---|
| 1 | Talc | Talcum Powder |
| 2 | Gypsum | Used in plaster of paris, deposited in lake and sea water |
| 3 | Calcite | Limestone and most shells contain calcite |
| 4 | Fluorite | Fluorine in Fluorite prevents tooth decay |
| 5 | Apatite | Infrequently used as a gemstone |
| 6 | Orthoclase | Common constituent of most granites |
| 7 | Quartz | Regularly found in passage tomb cemeteries in Europe |
| 8 | Topaz | The November birthstone. Can be found in the Ural mountains. |
| 9 | Corundum | Sapphire and Ruby are varieties of corundum. Twice as hard as Topaz. Sometimes used as an abrasive. |
| 10 | Diamond | Used in Jewellery and cutting tools. Four times as hard as Corundum. |
Moh chose the minerals because they were common or readily available at the time. The Moh's Hardness scale is not a linear scale unlike the scale of absolute hardness. So Orthoclase at 6 is not twice as hard as Calcite at 3, nor is the difference between talc and gypsum the same as the difference between Corundum and Diamond.
An absolute hardness scale gives a clearer idea of how hard the minerals are in relation to each other. As you can see in the scale below, Orthoclase is not even twice as hard as Apatite however diamond is four times harder than Corundum.
Absolute Scale of Hardness
- 1 Talc
- 3 Gypsum
- 9 Calcite
- 21 Fluorite
- 48 Apatite
- 72 Orthoclase
- 200 Topaz
- 400 Corundum
- 1600 Diamond
Knowing the hardness of some everyday items helps to use Moh's scale when identifying minerals and rocks.
Hardness of other easily accessible items
- 2.5 Fingernail
- 3 Old copper penny
- 4.5 Platinum
- 5.5 Knife blade
- 6-7 Glass
- 7 Hardened steel file
An item made from a harder material will mark or scratch a softer material on the scale so for example a knife blade will scratch anything with a hardness rating below 5.5 and a fingernail will scratch talc.
