What is the hardest substance in the human body?

The hardest substance in the human body is tooth enamel.

Enamel is the highly mineralized outer covering of a tooth's crown. About 96% of its composition is mineral, primarily hydroxyapatite, a crystalline calcium-phosphate compound. This dense structure allows enamel to withstand the repeated forces of biting and chewing.

Its hardness does not make enamel indestructible. Enamel is brittle and can crack, wear down, or dissolve when exposed to acids produced by oral bacteria or found in drinks such as soda and fruit juice. The critical pH for enamel demineralization is commonly given as about 5.5. Saliva can help replace some lost minerals, but the body cannot fully regrow mature enamel because it contains no living cells, blood vessels, or nerves.

Dentin is often confused with enamel because it forms much of the tooth beneath it, but dentin is softer. Bone is also strong and mineralized, yet enamel is harder. Enamel begins forming before a tooth erupts and is produced by specialized cells called ameloblasts.