Final answer:
The crown of a tooth is covered by enamel, which is the hardest material in the body and protects the underlying sensitive areas.
Step-by-step explanation:
The substance that covers the crown of a tooth is enamel. The crown is the portion of the tooth that is visible above the gum line and needs to be extremely hard to withstand biting and chewing forces. The enamel is the hardest material in the body and covers the dentin layer of the crown. It serves as a protective layer for the sensitive areas underneath, such as the pulp cavity with its blood vessels and nerves. Although the enamel is very durable, it is still susceptible to erosion from tooth decay, which occurs when bacteria in the mouth metabolize sugars and release acids.