In the crown of a tooth, the underlying dentin is covered by which substance, described as the hardest substance in the body?

1. A tooth has two main parts: the crown above the gum line and the root embedded in the jaw. 2. Both parts contain an inner pulp cavity surrounded by the bone-like tissue called dentin. 3. In the crown specifically, this dentin is covered by an outer layer of enamel, described as the hardest substance in the body. 4. Cementum instead covers the dentin in the root, pulp is the inner soft tissue rather than a covering layer, and the periodontal ligament holds the tooth in its socket rather than covering the crown's dentin. _Source: OpenStax Anatomy and Physiology (CC BY 4.0), Ch 23 "The Digestive System", section 23.3, p. 1103_