Diamond is the hardest naturally occurring substance found on Earth. But it is not the hardest substance. Wurtzite boron nitride (synthetic) and lonsdaleite (which comes from meteorites) are both harder.

Hardest known natural substance is Diamond:

Diamond is considered the hardest known natural substance. It is a form of carbon in which the carbon atoms are arranged in a crystal lattice structure. Here are some reasons why diamond is the hardest substance:

1. Structure:
- The crystal lattice structure of diamond is extremely rigid and tightly packed, making it difficult to break or scratch.
- Each carbon atom in diamond is bonded to four neighboring carbon atoms through strong covalent bonds, resulting in a three-dimensional network of interconnected atoms.

2. Carbon Bonds:
- The carbon-carbon bonds in diamond are exceptionally strong, making it difficult for the carbon atoms to move or break away from their positions.
- These strong bonds give diamond its exceptional hardness and durability.

3. Mohs Scale of Hardness:
- The hardness of a substance is measured on the Mohs scale of hardness, which ranges from 1 (softest) to 10 (hardest).
- Diamond has a Mohs hardness of 10, which means it is the hardest substance on the scale and can scratch all other materials.

4. Abrasion Resistance:
- Diamond is highly resistant to abrasion, meaning it can withstand wear and tear caused by friction or rubbing against other materials.
- This property makes diamond ideal for use in cutting tools, such as diamond blades and drill bits, which need to maintain their sharpness and durability.

5. Industrial Applications:
- The hardness and durability of diamond make it valuable in various industrial applications.
- It is used in the production of cutting and grinding tools, as well as in the manufacturing of high-precision instruments and machinery.

In conclusion, diamond is the hardest known natural substance due to its rigid crystal lattice structure, strong carbon-carbon bonds, and exceptional resistance to abrasion. Its hardness and durability make it highly sought after for both industrial and decorative purposes.