Apatite

Apatite is a group of phosphate minerals with high concentrations of OH−, F−, Cl− or ions, respectively, in the crystal.
Apatite is characteristic of biological systems. It is one of a few minerals produced and used by biological micro-environmental systems. It hardness defines 5 on the Mohs scale. Hydroxyapatite is the main component of tooth enamel and bone mineral.
Much bone material is in a relatively rare form of apatite. In this form most of the OH groups are absent, and there are many carbonate and acid phosphate substitutions.
Fluorapatite (or fluoroapatite) is more resistant to acid attack than is hydroxyapatite. In the mid-20th century it was discovered that communities whose water supply naturally contained fluorine had lower rates of dental caries. Fluoridated water allows exchange in the teeth of fluoride ions for hydroxyl groups in apatite. Similarly, toothpaste often includes a source of fluoride anions (e.g. sodium fluoride, sodium monofluorophosphate).