Tooth tissue. Coloured scanning electron micrograph (SEM) of tooth enamel (grey) and dentin (brown). The enamel is the outer covering of the crown (visible part) of the tooth. It is the hardest substance in the human body. It is composed of rows of calcium and phosphorus salts embedded in a protein matrix. Dentin makes up the majority of the tooth. It is composed of mineralised connective tissue. Magnification: x190 when printed at 10 centimetres tall.