How are diamonds formed?
Diamonds differ from rest of the gem minerals in that they were formed somewhere in the region between the lower part of the earth's crust and the beginning of the mantle. The current theory is that diamond crystallizes at an approximate depth of 100 to 200 kms beneath the earth's surface, from carbon, at very high temperatures & pressures. The diamond - bearing magma was then forced upto the earth's surface by explosive gas pressures in a volcanic type eruption.
The magma eventually cooled and solidified to form the present day kimberlite / lamproitic pipes which make up the bulk of the world's primary source. These pipes are thought to have originally extended above the surface as cone shaped hills or even as mountains. Over millions of years, these kimberlite / lamproitic hills were eroded away by the weathering action of wind and rain into low lying hillocks. The diamonds contained in the top section of the pipes were washed away to form alluvial deposits along river beds as in the case of south west Africa, along the marine terraces of the Namibian coast.