Given how white teeth are---and how tough---one might assume they are made of bone; this is not the case, though teeth and bones share a common component called hydroxylapatite. The chemical composition of teeth can be better understood by breaking down the four major tissues they are composed of: enamel, dentin, cementum and pulp.
Enamel---the whitish covering---is the hardest and most mineralized part of the teeth and of the entire body. About 96 percent of enamel consists of a mineral called hydroxylapatite, a form of which also makes up to 50 percent of bone. The other 4 percent is water and organic material. Because of its high concentration of mineral power, enamel is strong enough to withstand the stress of biting, chewing and grinding. However, that same trait makes enamel brittle and susceptible to cracking and chipping.
Dentin, a yellow-hued substance, makes up most of the tooth. It is responsible for giving the tooth its color. Dentin, which is less mineralized and less brittle than enamel---with 70 percent minerals (hydroxylapatite and calcium phosphate), 20 percent organic materials (primarily proteins) and 10 percent water---acts as a support for enamel. Because dentin is softer, it is more prone to decay.
Cementum, a yellowish substance covering the root of a tooth, is even softer than the enamel and the dentin. Hydroxypatite is its main mineral, comprising about 45 percent of it. Proteins, mainly collagen (the most abundant protein in humans), make up 33 percent of the cementum, while water is left with the remaining 22 percent. The role of the cementum is to help anchor the tooth to the jawbone and ensure its stability.
As the "nerve" of the tooth, the pulp's major role is to form dentin through biological cells at its outer surface called odontoblasts. The pulp is composed of blood vessels and nerves. Among the cells that make up the pulp are white blood cells like macrophages and T lymphocytes.
Researchers believe that fluoride---a version of the element found in toothpaste---is a prominent factor in tooth remineralization. Once fluoride gets onto the surface of the tooth, it attracts other minerals. Thus such exposure reduces tooth decay.