Diamond is not the hardest substance known, but the materials that are harder are much more rare. Researchers have reported that wurtzite boron nitride and lonsdaleite can be harder than diamond.
It is unlikely that you will find a mineral that is softer than talc. However, a few metals are softer. These include: cesium, rubidium, lithium, sodium, and potassium. You will probably never need to test their hardness.
Mohs Scale of Hardness Compared to Others
When Friedrich Mohs developed his hardness scale in 1812, very little information about mineral hardness was available. He simply selected ten minerals that varied in hardness and arbitrarily placed them on an integer scale from 1 to 10. It was a relative scale in which a mineral of unknown hardness could be tested against a group of ten index minerals to see where it positioned on the scale.
The Mohs scale has stood the test of time and has been widely used throughout the world for over 200 years – mainly because it is easy-to-do, inexpensive, and people quickly understand it. Other hardness tests have been devised, but none of them have seen such widespread use.
A “Mohs hardness” is a relative integer-scale comparison of “resistance to being scratched.” Most other hardness scales use “resistance to indentation under a stylus to which a specific amount of pressure is applied for a specific length of time.” Although these tests differ from Mohs hardness in their procedure, they are all tests of the resistance to atoms being dislodged from their positions by pressure against the surface of a mineral specimen.
One of these scales is the Vickers Hardness Scale. In the Vickers test, the size of the indentation is microscopically estimated and used to calculate a hardness value. The Vickers hardness values form a continuous scale which provides more information about the hardness of minerals when compared to the integer values of the Mohs scale. A table comparing the Mohs scale minerals to their Vickers hardness is shown here along with a graph of the data. The graph shows that in terms of Vickers hardness, the gaps between the integer values of the Mohs scale are not uniform in width. In addition the gaps between minerals of higher Mohs hardness are much broader than those between the softer minerals. In terms of Vickers hardness, diamond is enormously harder than corundum.
| Mineral Hardness Scales | ||
| Mineral | Mohs | Vickers (kg/mm2) |
| Talc | 1 | 27 |
| Gypsum | 2 | 61 |
| Calcite | 3 | 157 |
| Fluorite | 4 | 315 |
| Apatite | 5 | 535 |
| Orthoclase | 6 | 817 |
| Quartz | 7 | 1161 |
| Topaz | 8 | 1567 |
| Corundum | 9 | 2035 |
| Diamond | 10 | 10000 |