The mineral fractions of bones and teeth are non-stoichiometric apatite crystals. Their morphology, dimensions, composition and reactivity are adapted to their biological function. Bone apatites are made of very reactive nanocrystals possessing, on their surface, a structured hydrated layer responsible for their biological properties. This unstable layer becomes progressively and unavoidably transformed into a relatively inert apatite lattice. In spite of very different time periods, the laws that determine the biological and diagenetic evolution of bone mineral are the same, especially in processes involving dissolution–re-precipitation phenomena.