Bones and teeth consist of an inorganic calcium phosphate mineral approximated by hydroxylapatite and matrix proteins. The physical and chemical properties of these “bioapatite” crystals are different from those of geologic hydroxylapatite because of the way they are formed, and these unique properties are required for fulfilling the biological functions of bones and teeth. Recent biochemical studies provide insight into the factors controlling the formation and growth of bioapatite crystals and how alteration in the mineralization process can lead to diseases such as osteoporosis. New spectroscopic and microscopic techniques are enabling scientists to characterize changes in crystal properties in these diseases, providing potentially fruitful areas of collaboration among geochemists, mineralogists, and biological researchers and offering hope for the development of novel therapies.