Human tooth fractured enamel, coloured scanning electron micrograph (SEM). Fracture in a tooth revealing the enamel rod morphology. Tooth enamel is the highly mineralized covering of the crown of a tooth. It is the hardest and most highly mineralized substance of the body. Ninety-six percent of enamel consists of mineral, with water and organic material composing the rest. Enamel is a translucent substance composed of parallel rods (or prisms) of highly calcified material cemented together by a hard, calcified substance. The basic unit of enamel is called an enamel rod or enamel prism which is a tightly packed mass of hydroxyapatite crystals (also called hydroxyapatite crystals) in an organized pattern. The rods are arranged perpendicular to the surface of the enamel. Hydroxylapatite is a crystalline calcium phosphate. Also enamel has two unique classes of proteins called amelogenins and enamelins. Magnification: x400 when shortest axis printed at 25