Hydroxyapatite is the hydrated calcium phosphate mineral — Ca₁₀(PO₄)₆(OH)₂ — that gives bones and teeth their hardness: the inorganic scaffolding of the skeleton, with collagen providing the flexible matrix around it. Tooth enamel is almost pure hydroxyapatite, the hardest substance in the body; bone is roughly two-thirds mineral. Nutrition writes directly onto this mineral: calcium, phosphorus, and vitamin D intakes in childhood build its density, while deficiency and ageing leach it away as osteoporosis. Food-grade hydroxyapatite appears as an anti-caking agent and calcium source in supplements, and nano-hydroxyapatite is the remineralising ingredient in modern toothpastes — rebuilding enamel crystal by crystal.