The hardest rock type is diamond, which is actually a form of carbon that has undergone metamorphic processes under high pressure and temperature conditions deep within the Earth. Diamonds are renowned not only for their hardness, making them invaluable in industrial applications for cutting, grinding, and drilling, but also for their brilliance and beauty in jewelry. Their hardness is measured at 10 on the Mohs scale, the highest rank, making diamonds resistant to scratching and abrasion. This unique property, combined with their aesthetic appeal, contributes to their long-standing value and popularity.