Diamond Diamond is the hardest naturally occurring substance currently found on Earth. Diamond has a Mohs hardness of 10, and its absolute hardness is 4 times that of corundum and 8 times that of quartz. There are two main methods for synthetic diamond, high temperature and high pressure method and chemical vapor deposition method. Diamonds are used in a wide range of applications, such as crafts, cutting tools in industry, and precious gemstones.