The 10-point scale of mineral Hardness. Devised by Friedrich Mohs, a German mineralogist, in the 19th century.

The comparative scale of hardness is as follows:
1)    Talc
2)    Gypsum
3)    Calcite
4)    Fluorite
5)    Apatite
6)    Feldspar
7)    Quartz
8)    Topaz and Beryl
9)    Corundum
10)  Diamond

I find it is better to describe this as scratch resistance, since hardness might imply Toughness, which is very different. Diamond resists scratching more than any other material, (the hardest gem material by far). But it is not the toughest gemstone (not as breakage resistant).

The diamond is the hardest of all known substances, natural or man-made.

WP Glossary Term Usage