Teeth and Diseases Affecting Your Teeth Teeth are numerous, hard tissue structures in the mouths of humans. In humans the teeth are composed of nan inner layer of dentin that, above the gum line, is covered by a layer of enamel. Called the crown, this part of the tooth is extremely hard. The root portion of the tooth is covered with a bonelike substance called cementum. The root is embedded in the jawbone and lies in a socket, where the cementum is attached to the adjacent bone by a periodontal membrane, consisting of fine consecutive tissue fibers. The tooth is also attached to the gum tissue