Final answer:
Diamond is made entirely of carbon, the hardest known natural substance, forming a strong three-dimensional network with a high melting point of around 4400 °C.
Step-by-step explanation:
Diamond is made entirely of the element carbon. It is known for being the hardest known natural substance, which is attributed to its unique structure where each carbon atom forms four strong single bonds to four other carbon atoms. This structure is in a tetrahedral arrangement (sp³ hybridization), resulting in a giant molecule with a robust three-dimensional network. These strong carbon-carbon bonds contribute to diamond's remarkable properties, which include very high melting points, approximately 4400 °C, and the ability to resist deformation and shattering under large stresses.