Diamonds are formed in geologically stable regions of the continents, at depths of 100-200 km, where the temperature reaches 1100-1300 degrees Celsius, and the pressure reaches 35-50 kilobars. Such conditions favor the transition of carbon from graphite to another modification - diamond, which has a cubic structure closely packed with atoms. After spending billions of years at great depths, diamonds are carried to the surface by kimberlite magma during volcanic explosions, thus forming primary diamond deposits - kimberlite pipes.
Currently, diamonds are mined from two types of deposits: primary (kimberlite and lamproite pipes) and secondary - placers.
Diamond mining is a complex and time-consuming process that requires significant financial investments at the initial stages. On average, about 1 carat of diamonds from primary deposits and 3-5 from alluvial deposits are mined from one ton of rock.
More information https://gemscience.net/geological-origin-of-natural-diamonds/