Geologists believe that the diamonds in all of Earth's commercial diamond deposits were formed in the mantle and delivered to the surface by deep-source volcanic eruptions. These eruptions produce the kimberlite and lamproite pipes that are sought after by diamond prospectors.
Most of these pipes do not contain diamond, or contain such a small amount of diamond that they are not of commercial interest. However, open-pit and underground mines are developed in these pipes when they contain adequate diamonds for profitable mining. Diamonds have also been weathered and eroded from some of these pipes. Those diamonds are now contained in the sedimentary (placer) deposits of streams and coastlines.
The formation of natural diamonds requires very high temperatures and pressures. These conditions occur in limited zones of Earth's mantle about 90 miles (150 kilometers) or more below the surface, where temperatures are at least 2000 degrees Fahrenheit (1050 degrees Celsius). The critical temperature-pressure environment for diamond formation and stability is not present globally. Instead it is thought to be present primarily in the mantle beneath the stable interiors of continental plates.
Diamonds formed and stored in these "diamond stability zones" are delivered to Earth's surface during deep-source volcanic eruptions. These eruptions tear out pieces of the mantle and carry them rapidly to the surface. This type of volcanic eruption is extremely rare and has never been observed by modern humans.
Is coal involved? Coal is a sedimentary rock, formed from plant debris deposited at Earth's surface. It is rarely buried to depths greater than two miles (3.2 kilometers). It is very unlikely that coal has been moved from the crust down to a depth well below the base of a continental plate. The carbon source for these mantle diamonds is most likely carbon trapped in Earth's interior at the time of the planet's formation or delivered to great depths by subduction.