Enamel is 96% inorganic, mainly hydroxyapatite
crystals, with 4% water and fibrous organic
material. It is the hardest substance in the
body and covers the exterior surface of the
crowns only. The enamel consists of hexagonal
prisms or rods of hydroxyapatite crystals held
together by a cementing organic matrix.
Enamel is formed by ameloblasts within the tooth bud before eruption.
It is capable of only very limited repair when damaged, once the
tooth has erupted.