Human tooth has a two-tiered structure, crown that is exposed and root that is buried in gums. The crown and root consist of three layers.

The pulp is the part in the center of a tooth. This contains large nerve trunks and blood vessels. And the pulp provides dentin, feels pain and performs the immune defenses. "Pulling out the tooth nerve" means removing the pulp.

The second layer of tooth is called dentin. About 70% of dentin consists of the mineral hydroxylapatite, 20% is organic material and 10% is water. Dentin rates approximately 5 or 6 on the Mohs scale of mineral hardness. That is as hard as a steel knife.

The most outer part of the root is cementum. About 60% of cementum consists of the mineral hydroxylapatite, 25% is organic material and 15% is water.

The surface of the crown is called enamel. About 96% of enamel consists of the mineral hydroxylapatite and 4% is organic material and water. Enamel is the hardest and most highly mineralized substance in the human body and rates approximately 6 or 7 on the Mohs scale of mineral hardness. That is as hard as crystal but is fragile and easy to peel off. So Enamel has to be supported by the under layer, dentin. Before the tooth erupts into the mouth, the ameloblasts that provide enamel had been disappeared and enamel has no way to regenerate itself.