What is the hardest natural substance on Earth?

Diamonds are recognized as the hardest natural substance on Earth due to their unique crystal structure and the strong covalent bonding between carbon atoms. This arrangement allows diamonds to withstand high levels of pressure and resist scratching by other materials. The hardness of a mineral is measured by the Mohs scale, where diamonds rank at the top with a rating of 10, indicating that they can only be scratched by other diamonds.

In contrast, gold, iron, and ruby possess significantly lower hardness ratings. Gold and iron are metals, which are not categorized as hard substances compared to minerals like diamond. Ruby, while a very hard gemstone made of corundum with a Mohs hardness of 9, still cannot match the durability of diamond. Therefore, the unique properties and structure of diamonds make them the hardest natural substance on Earth.