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The Mohs hardness scale was developed in the 1800s by Friederich Mohs. He was a German geologist and mineralogist.
Around 1835, he started classifying minerals by their physical characteristics. Traditionally, classification had been done based on the mineral’s chemical composition. But, both Theophrastus and Pliny the Elder had compared the relative hardness of minerals known to them in the ancient world. They knew that diamond could scratch quartz, meaning the diamond was harder. This became the basis of the hardness scale developed by Mohs.
He picked ten minerals and assigned them numbers based on the ease or difficulty with which one can be scratched by another. The hardest mineral, diamond was given a value of 10 and softer minerals, such as talc, were given the very low value of 1. Mohs' hardness is a measure of the relative hardness and resistance to scratching between minerals.
Since hardness depends upon the crystallographic direction (ultimately on the strength of the bonds between atoms in a crystal), there can be variations in hardness depending on the direction one measures this property.
The Mohs Scale isn’t super precise, but it’s handy for field geologists, who use the scale to roughly identify minerals using scratch kits. Scratching kits can be bought or you can make one if you’re familiar with the minerals and their hardness. A “hardness pick” can also be used. These picks have sharp metal points that you can use for very accurate testing. The sharp picks are easy to use and either produce a scratch if they are harder than the specimen being tested or leave behind a tiny streak of metal if they are softer.
Nerdy Note: Mohs Scale is just one of several factors gemologists use to determine durability of a mineral or gemstone.