What is the hardest natural substance?

Diamond is carbon, and so is the graphite in a pencil. The difference is entirely in the arrangement: in diamond every carbon atom is bonded to four others in a rigid three-dimensional lattice with no weak direction, while graphite stacks its atoms in flat sheets that slide over each other, which is why one cuts glass and the other leaves a mark on paper.

Hardness here means resistance to being scratched, and diamond sits at the top of the Mohs scale at 10. The scale is deceptive, though, because it only ranks - it does not measure distance. Diamond is roughly four times harder than corundum at 9, a bigger gap than the whole of the rest of the scale put together.

Hard is also not the same as tough. A diamond will scratch anything, and a sharp tap along a cleavage plane will split it - cutters rely on exactly that. They form 150 kilometres or more below the surface over enormous spans of time, and reach us only because certain violent volcanic eruptions carry them up fast enough that they do not have time to turn back into graphite on the way.