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Make sure you know this when buying something hard

Symbol: HR / Base data: hardness.
A type of indentation hardness. A unit of hardness shown by the indentation depth when denting materials with a diamond cone or steel ball at a constant load.

Unit of hardness discovered by applying force

This is an index that shows the hardness of materials such as metal, and force is applied to that material to check its hardness. This method, discovered by someone called Rockwell in 1919, uses a diamond cone or steel ball to make indentations. It is useful for displaying the hardness of steel knives.