Tooth enamel, dentine, and cementum are calcified structures. Enamel covers the crown of the tooth. The enamel composed of approximately 96% inorganic apatite crystals and 4% organic material and water, and it is a highly mineralized and hardest tissue in the body. The organic component of enamel is the protein enamelin found between and on the crystals, which aids enamel permeability. This is similar to the protein keratin found in the skin.