The hardest rock is diamond, which is an allotrope of carbon. Diamonds form under high-pressure conditions deep within the Earth's mantle over millions of years. Their hardness is measured on the Mohs scale, where diamond scores a perfect 10, making it the hardest naturally occurring material known. This extraordinary hardness enables diamonds to be used in various industrial applications, including cutting, grinding, and drilling. Additionally, diamonds are prized as gemstones for their beauty, brilliance, and durability. Their unique structure and formation contribute to their allure and significance in jewelry and culture around the world.