The quality of being physically firm and difficult to break or penetrate, or being emotionally tough and unfeeling.
From Old English 'heard,' related to German 'hart.' The physical sense referred to resistance to cutting or breaking, while the emotional sense developed in Middle English as people began using material properties as metaphors for character.
Diamonds are the hardest natural material we know, which is why gemologists use the Mohs hardness scale—a system that literally measures how resistant substances are to scratching!
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