The hardest natural substance on Earth is diamond. Diamonds are formed under high-pressure, high-temperature conditions deep within the Earth's mantle over millions of years and are composed of carbon atoms arranged in a crystal structure that gives them their exceptional hardness. This property makes diamonds ideal for various industrial applications, such as cutting, grinding, and drilling. In addition to their practical uses, diamonds have significant cultural and emotional value, often symbolizing love and commitment, especially in the form of engagement rings. Their brilliance and rarity further contribute to their desirability.