Hardness of Gemstones

All materials have a 'hardness' – its ability to resist scratching – and this can be given a numerical value from the Mohs' Scale:

Value        Defining Mineral
1                talc
2                gypsum
3                calcite
4                fluorite
5                apatite
6                orthoclase
7                quartz
8                topaz
9                corundum
10              diamond

This scale is not regular i.e. the intervals between the numbers are not equal, the most striking example of this is that diamond at number 10 is not simply one factor higher than corundum (ruby and sapphire) but is 140 times harder1.