Tooth Enamel

The hardest naturally occurring substance in the human body is tooth enamel.
But it’s not the hardest substance known to scientists – and neither are diamonds (they actually rank 7th behind some natural and some synthetic materials).
The hardest substance found in nature (or in extraterrestrial nature) is Lonsdaleite, which is carbon (graphite) discovered from the aftermath of a meteoric collision with the earth. On impact, the pressure inside the meteor causes the graphite to compress into a crystalline hexagonal lattice (diamonds have a cubic lattice structure) which can make it 58% harder than naturally occurring diamonds. Although most Lonsdaleite is impure, which softens the entire substance, impurity-free it makes quite an impact.
The hardest material known to us today is actually a synthetic array of carbon, created in laboratory and called graphene. Graphene is a defect-free network of carbon atoms bound into a perfectly hexagonal arrangement – a carbon lattice that is a single atom thick.