Hardness (3.04)
Diamond is the hardest natural known substance on earth and so naturally Friedrich Mohs based his hardness scale on just that.
Mohs’ Scale
10) diamond
9) corundum (ruby/sapphire)
8) topaz
7) quartz
6) feldspar
5) apatite
4) fluorite
3) calcite
2) gypsum
1) talc
The divisions were chosen arbitrarily and are unequal. For example the difference in hardness between 8 and 9 is much greater than the difference between any of the lower numbers. The difference in hardness is greater between 9 and 10 than between 1 and 9. Needless to say diamond is an extremely hard substance capable of resisting scratching by any other substance on earth except for diamond itself.
Toughness (3.05)
We’ve all heard the old saying “A diamond is forever.” Forever is an overstatement but since diamonds are extremely hard and do resist any kind of scratching, this would be a likely assumption. Diamonds will often break, given the right circumstances. How could an extremely hard substance break? It’s so hard that it’s brittle. Actually diamond’s toughness is rated “good” (not “excellent” like the gemstone jade). Remember that there is a significant difference between hardness and toughness. Hardness is simply the resistance to scratching and toughness is the resistance to breakage, to powdering or to flattening under repeated pounding.♦