The crown of each tooth is covered with enamel, which protects the underlying dentine. Enamel is the hardest substance in the human body, even harder than bone. This is because it is the most mineralised substance in the body, made up of crystalline calcium phosphate (Hydroxylapatite). It is as hard as crystal.
Enamel is the only tissue that has no living cells. Because it is not alive, it can’t repair itself from decay or damage.