--------- is the hardest substance that occurs naturally.

Hint: To solve this problem we should understand about carbon, various forms of carbon occurring in nature, their bonding, including other properties and further information about them and hence by using this knowledge we can approach the answer.

Complete step by step solution:
> Carbon(C) is a chemical element widely distributed in nature having atomic number 6, carbon makes up about 0.025 percent of Earth’s crust. It is nonmetallic and tetravalent making four electrons available to make up covalent chemical bonds. It belongs to the 14th group of the periodic table.
> Chemical properties of carbon are combustion, Oxidation, Addition reaction and Substitution reaction.
> Carbon can form allotropes, (Allotropes are the structurally different forms of the same element) which happens because of its valency. Examples of several different carbon allotropes are diamond, graphite, buckminsterfullerene.
> Diamond is a well-known allotrope of carbon. It has applications in industries and jewellery due to its hardness and high dispersion of light. It is the hardest natural mineral. The dominant industrial use of diamond is cutting, drilling, grinding and polishing. Specialized applications include use in laboratories as contaminants for high pressure experiments.
> Diamond is the hardest substance that occurs naturally.

Hence, the correct answer is Diamond.

Note - Chemical properties of carbon are Oxidation, in which it combines with oxygen to produce carbon dioxide and carbon monoxide. When we talk about reactivity, carbon does not dissolve in, or react with, water or acids. Carbon forms more compounds than all other elements combined; several million carbon compounds are known ${C_{60}}$ Carbon also occur in a newly discovered form known as fullerenes. Fullerenes have similar structure as of graphite.