What is the hardest natural substance on Earth?

Diamonds are known as the hardest natural substance on Earth because of their unique crystal structure. They are formed under high-pressure and high-temperature conditions deep within the Earth's mantle, where carbon atoms bond in a specific tetrahedral arrangement. This strong covalent bonding gives diamonds their exceptional hardness, making them able to scratch virtually all other materials. This characteristic is the reason diamonds are commonly used in cutting tools and abrasives.

The other materials listed have their own properties but do not match the hardness of diamonds. Gold, while a precious metal, is relatively soft compared to diamonds. Iron is a strong metal but can be easily scratched by materials with a higher hardness like diamonds. Granite, a type of igneous rock, is hard due to its mineral composition, but it is still far less hard than diamond. Therefore, the recognition of diamonds as the hardest natural substance is based on both their physical properties and the scientific understanding of their formation and structure.