Multiple choice Which of the following is the hardest substance in the body? Hair Bone Nail Enamel Brain Reveal answer Fill a bubble to check yourself D Correct answer Explanation Tooth enamel is the hardest substance in the human body, composed primarily of hydroxyapatite crystals. It measures 5 on the Mohs scale of mineral hardness, harder than bone (which is around 3-4). Enamel protects teeth from wear and decay.