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For more on marking an answer as the "Best Answer", please visit our FAQ.Diamond. From the practical point of view, the Mohs scale (Friedrich Mohs, the German mineralogist 1773-1839) is still in use. It's so called "scratch hardness" (A scale for classifying minerals based on relative hardness, determined by the ability of harder minerals to scratch softer ones. The scale includes the following minerals in order from softest to hardest: 1.talc,2.gypsum,3.calcite,4.fluorite,5.apatite,6.orthoclase feldspar,7.quartz,8.topaz,9.corundum,10.diamond. Higher-grade diamonds are used to cut lower-grade diamonds, although not everything is that simple. Mohs' is purely ordinal scale with, for example, corundum being twice as hard as topaz, but diamond, almoust four times as hard as corundum.
The mnemonic traditionally taught to geology students to remember this table is "The Girls Can Flirt And Other Queer Things Can Do".
There are also some other scales (Indentation hardness, Rebound hardness scales).