Diamonds are expensive because the supply of diamonds on the world market is restricted by DeBeers. The price of diamonds would be much lower if the supply were not artificially suppressed.
Among gems, diamonds are the most common "precious" stones on Earth. The perceived value of diamonds is the result of a restricted supply and savvy marketing by DeBeers. Prior to the 1870s, diamonds were rare, but the discovery of African diamond mines changed that. DeBeers enlisted Hollywood stars in the 1930s to spur demand among the general public for its newly discovered surfeit and developed the slogan "A Diamond is Forever," which is still in use more than 80 years later.