The hardest known substance on Earth is diamond. Formed under extreme conditions of temperature and pressure deep within the Earth's mantle, diamonds are made of carbon atoms arranged in a crystal lattice structure. This unique arrangement gives diamonds their impressive hardness, making them valuable not only in jewelry but also in industrial applications such as cutting, grinding, and drilling. Their ability to refract light beautifully also contributes to their desirability in adornments. Diamonds have fascinated humans for centuries, symbolizing power, luxury, and eternity.