Which of the following is the hardest natural substance? 

Diamond 

The correct answer is option 1. Diamond.

Among the options provided, diamond is the hardest natural substance. Diamond is a form of carbon where the carbon atoms are arranged in a crystal lattice structure, known as a diamond cubic structure. This arrangement results in extremely strong covalent bonds between the carbon atoms, making diamond exceptionally hard. Diamond is renowned for its hardness, durability, and high refractive index, making it valuable for various industrial applications, including cutting tools, abrasives, and jewelry. So, the correct answer is Diamond.