hardness of rock

An indication of scratch resistance based on the relative hardness of 10 common minerals which, in order of increasing hardness, are: 1 talc, 2 gypsum, 3 calcite, 4 fluorite, 5 apatite, 6 orthoclase, 7 quartz, 8 topaz, 9 corundum, 10 diamond. 

Also: Moh’s scale of hardness