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Even a diamond-tipped drill has to work harder against one part of your body than against everyday steel.
Tooth enamel is the hardest substance in the human body because it is made almost entirely of tightly packed mineral crystals, arranged in a dense structure that resists scratching and wear far better than bone or any other tissue. This toughness is essential since teeth spend a lifetime biting and grinding food.
Bone is strong and can heal itself over time, but it is softer and more flexible than enamel, and fingernail keratin, while durable enough to protect fingertips, is much softer still, so neither can match enamel's mineral hardness.
Unlike bone, enamel has no living cells inside it, so once a cavity or chip damages it, the body cannot regrow or repair that enamel, which is exactly why dentists have to fill or cap damaged teeth instead of letting them heal naturally.
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