is diamond the hardest substance in the world

Yes, diamond is the hardest known natural material on the Mohs scale of mineral hardness. It scores a 10 on this scale, which is the highest rating possible. The hardness of diamond is due to its unique crystal structure, where carbon atoms are bonded together in a strong and rigid lattice known as the tetrahedral structure. This arrangement of bonds makes it extremely difficult for other materials to scratch or cut through diamonds.

It's worth noting that while diamond is the hardest natural substance, there are synthetic materials, like some forms of cubic boron nitride, that can rival or even surpass the hardness of diamond in certain applications.