I have just watched a fascinating video by TED-Ed on the evolution of teeth (lesson by Peter S. Ungar).
Teeth have the remarkable property that they are both hard and tough. Few substances are like that – for instance, glass is hard but not tough, while leather is tough but not hard. The hardness is due to an outer layer of enamel, produced by cells called ameloblasts, while dentine, produced by odontoblasts, confer the toughness. Dentine can be repaired when damaged, but enamel cannot be regenerated as the ameloblasts die after depositing the material. This is why tooth decay can be a serious problem.