Although diamond is known as the world’s hardest natural material and has been assigned a hardness of 10 on the Mohs hardness scale, that information is an oversimplification. Diamond crystals vary in hardness by direction.
| 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 |
| Mineral Hardness ScalesMineralMohsVickers (kg/mm2)Talc127Gypsum261Calcite3157Fluorite4315Apatite5535Orthoclase6817Quartz71161Topaz81567Corundum92035Diamond1010000 |
The direction of greatest hardness is parallel to the octahedral crystal planes. When diamond crystals are being cut and polished into gems, it is very difficult to cut them in that direction with a diamond saw. So instead of using a diamond saw or the traditional practice of breaking them by cleaving, much of this work is now done by laser sawing.
Facets cut parallel to the octahedral crystal direction are also difficult to polish, so cutters either change direction or risk leaving a “lizard skin” texture on the facet.
The softest direction in a diamond crystal is parallel to the cubic planes. The best polishing is done on facets that are parallel to that direction. Although that is the softest direction in a diamond, the hardness is several times harder than corundum, the second-hardest mineral of the Mohs hardness scale.