Diamond is currently thought to be the hardest natural material on Earth, having a hardness of ten out of ten on the Mohs scale of mineral hardness. Diamond is made up of carbon atoms which share strong covalent bonds (where electrons are shared between atoms) and are equally spaced in a lattice arrangement. These atoms cannot move, which is what makes diamonds so hard.
However, computer simulations have shown that recently discovered rare natural materials wurtzite boron nitride and lonsdaleite are respectively 18% and 58% harder than diamond! However, they have yet to be tested in laboratory settings due to their rarity, and so have yet to claim the title of the Earth’s hardest material.
Dominique Sleet, Science Museum