What is the hardest known natural material on Earth?

According to both Mohs and Vickers scales, diamond is the hardest natural mineral on Earth. It is a solid form of carbon with an extremely rigid arrangement of atoms. It has a high refractive index and the highest thermal conductivity of any natural element. Most gem-quality diamonds form at a depth of 100 miles below the Earth’s surface, where temperatures (and pressure) conditions are incredibly high.

Source: Rank Red