Given the task definition and input, reply with output. Given a scientific passage and an answer, generate a question for the given answer.

Passage: Diamond is a form of carbon in which each carbon atom is covalently bonded to four other carbon atoms. This forms a strong, rigid, three-dimensional structure (see Figure below ). Diamond is the hardest natural substance, and no other natural substance can scratch it. This property makes diamonds useful for cutting and grinding tools as well as for rings and other jewelry (see Figure below ). Answer: carbon
The hardest natural substance, diamond is a form of what element?