What is the hardest natural substance on Earth?

Diamond is recognized as the hardest natural substance on Earth due to its unique structure and bonding. Diamonds are formed under extreme pressure and high temperatures deep within the Earth's mantle. Their atoms are arranged in a strong tetrahedral lattice structure, where each carbon atom is covalently bonded to four other carbon atoms. This arrangement results in exceptional hardness, making diamonds capable of scratching or resisting damage to nearly all other materials.

Gold and iron, while they are valuable and important in various industrial applications, are significantly softer than diamonds and can be scratched or deformed under pressure. Granite, although a tough and durable rock composed mainly of quartz and feldspar, does not match the hardness of diamonds, as it is primarily known for its compressive strength rather than its scratch resistance.