Name an allotrope of carbon which is among the hardest substances known.

1. Allotrope: Allotrope refers to one or more physical forms of a chemical element that occurs in the same physical state.
2. Carbon has 3 allotropes: Diamond, graphite and fullerene.
3. Diamond is the hardest allotrope of carbon$\left(\mathrm{C}\right)$ because It is a strong and rigid three-dimensional structure.
4. Diamond is used for cutting many substances. It is so hard that no other substance can cut or break diamond. Only diamond can cut diamond.

Thus, Diamond is the hardest allotrope of Carbon.