Tooth enamel is the hardest substance the human body produces.

Enamel, the outer layer covering each tooth, is composed of about 96% mineral content — mostly a crystalline form of calcium phosphate called hydroxyapatite — making it harder than bone and harder than any other tissue the body makes. That hardness lets teeth withstand a lifetime of biting and grinding forces.

Enamel’s toughness comes at a cost: unlike bone, it contains no living cells and cannot repair or regenerate itself once damaged. Acid from food, bacteria, and drinks can gradually wear it away, which is why enamel erosion and cavities, once they form, require a dentist’s intervention rather than the body’s own healing processes.