Tooth enamel rods (formerly known as enamel prisms), coloured scanning electron micrograph (SEM). Enamel is the hardest biological tissue known in the body. It forms a very hard, thin, translucent layer of calcified tissue. Enamel is made up of hydroxyapatite crystals that are organised into tightly packed bundles known as rods or prisms. Magnification: x3200 when printed at 10cm wide.