Mohs Scale
The most commonly used scale of relative hardness of minerals :
Diamond 10
Corundum 9
Topaz 8
Quartz 7
Feldspar 6
Apatite 5
Fluorite 4
Calcite 3
Gypsum 2
Talc 1
The divisions are not equal, having been chosen arbitrarily by the German mineralogist Mohs. The difference between 9 and 8 is considerably greater than between any of the lower numbers. Between 9 and 10 the difference is greater than between 9 and 8.