Mohs' scale
The Mohs' scale of hardness was described in 1822 by Frederick Mohs, an Austrian minerologist, who developed the scale based on scratch tests performed by miners. It is unknown who originally came up with the technique, but it is attributed to Mohs since he first published it.
The scale determines the hardness of minerals based on their resistence to abrasion--it tests how easily one material might scratch another, and in particular the resistence between minerals. Some minerals may be able to scratch others more easily than the reverse case.
Unfortunately, while Mohs based his scale off of ten of the most readily found minerals of his time, the scale is not even.
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Diamond, for example, is extremely harder than corundum; flourite is not much harder than calcite. However, there are many other minerals that fit within the scale that can provide a better analysis of what the change in hardness actually "means".