Mohs' scale of mineral hardness was created by the German mineralogist Friedrich Mohs in 1822 to measure the relative hardness or scratch resistance of various minerals. He based it on ten readily available minerals. As it is an ordinal scale, you have to compare two minerals to decide which is harder. The scale is neither linear nor logarithmic. For example, corundum is twice as hard as topaz, but diamond is almost four times as hard as corundum.
The hardness of a material is measured against the scale by finding the hardest material that that the given material can scratch, or the softest material that can scratch the given material.
To give some examples from everyday life, a fingernail has a hardness of 2; a copper penny, about 3; a knife blade, 5; window glass, 5.5; and a steel file, 6.5.
Here are the Mohs scale ratings for 126 types of gemstones:
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