The hardest compound known to man is diamond, which is made up of pure carbon atoms arranged in a cubic crystal structure. Diamonds are extremely hard because the carbon atoms in their structure are very strongly bonded together, making them resistant to scratching, abrasion, and other forms of wear and tear. In fact, diamonds are the only known material that can scratch other diamonds. The hardness of diamonds is also why they are used in industrial applications such as cutting tools, abrasives, and polishes.