Mohs' scale of mineral hardness
The Mohs' scale of mineral hardness was created in 1812 by the German mineralogist Friedrich Mohs.
The table is based on the ability of a harder material to scratch a softer material.
This method is very old and has been first mentioned by Theophrastus in his treatise On Stones, circa 300 BC, followed by Pliny the Elder in his Naturalis Historia, circa 77 AD.
The Mohs' scale of mineral hardness specifies the range from 1 (softest) to 10 (hardest) where the softest mineral is Talc and the hardest is diamond.
Since this scale was invented there have been reports of materials harder than diamond, so the Mohs' scale may be changed in the future.