Diamond is the hardest of all the minerals. According to Mohr's hardness scale, it has a hardness of 10. The carbon atoms in diamond are shared by a strong covalent bond. They are formed in the mantle at a depth of about 150 km and beyond that, where the temperature and pressure condition is extremely high. These diamonds are carried to the near-surface region with the help of kimberlite pipes due to the deep volcanic eruption that is initiated.<u>Diamonds are very expensive because they are formed at huge depth and their cost of extraction is really high.</u> <u>They are very rare so they are really valuable</u>.
Thus, diamonds are used as a gem because of their hard and adamantine luster.
