Diamonds are supposed to formed about 90 miles below the surface of the earth in the mantle. Therefore, the weight of the earth overhead them produces the pressure to form the diamond. The rocks weight overhead the mantle outcomes in high pressure and diamonds essential high temperatures and pressure to form as of carbon.Diamonds form in the mantle in definite precincts where situations are right for them to do so. There just chances to be the right critical temperature-pressure-mineral environment in the mantle in definite areas for their formation.