A diamond is a solid form of the element carbon. While graphite is the most stable form of carbon at the Earth’s surface, a diamond will not spontaneously convert back because a significant energy barrier must be overcome. The extreme conditions necessary to force carbon atoms into the dense, three-dimensional tetrahedral lattice of a diamond exist only in rare pockets deep within the planet. Understanding the formation of this mineral requires examining the three factors that must converge: the source of the carbon, the physical forces, and the long-term geological stability of the environment.
The Necessary Ingredient: Carbon
The carbon atoms that become diamonds originate from two distinct sources. One source is primordial, consisting of carbon locked away within the Earth’s mantle since the planet’s formation billions of years ago. This ancient carbon is part of the deep-seated material known as peridotite.
The second primary source involves recycled surface carbon, which is transported deep into the mantle via plate tectonics. When oceanic crust slides beneath a continental plate—a process called subduction—it carries carbonates and other carbon-containing minerals. These materials are exposed to increasing pressure and temperature as they descend, eventually contributing carbon to the mantle fluid from which diamonds crystallize.
The carbon atoms must be dissolved in a carbon-bearing fluid or melt. This fluid acts as the transport mechanism, allowing the carbon atoms to move and bond into the unique, compact crystal structure that defines a diamond.
Extreme Conditions: Pressure and Temperature
Diamond formation is only possible within a defined zone of high pressure and high temperature, known as the diamond stability field. Below these conditions, carbon atoms arrange themselves into graphite.
The required pressure for natural diamond formation ranges between 4.5 and 6 Gigapascals (GPa). This is equivalent to 45,000 to 60,000 times the atmospheric pressure at the Earth’s surface. This colossal force pushes the carbon atoms into the tight, three-dimensional tetrahedral lattice.
Simultaneously, the temperature must be extremely high, generally falling within the range of 900°C to 1,300°C. This heat provides the necessary energy for the carbon atoms to rearrange into the new crystal structure. These combined requirements are met only at depths of 90 to 120 miles (140 to 190 kilometers) beneath the surface.
The Geological Location and Timeframe
The location where these conditions are met and maintained is within the thick, stable roots of ancient continents, known as cratonic keels. These keels are the deepest parts of the continental lithosphere, extending hundreds of kilometers into the mantle. They are characterized by a low geothermal gradient, which allows the pressure-temperature environment to remain within the diamond stability field.
This infrastructure provides the necessary long-term stability for diamond growth. Most natural diamonds have ages ranging from 1 billion to over 3.5 billion years, indicating a slow, prolonged growth period.
Carbon-bearing fluids, rich in compounds like water, carbon dioxide, or methane, play a role in this lengthy process. These fluids migrate through the mantle rock and allow the dissolved carbon to crystallize onto a growing diamond seed. The combination of a stable cratonic environment and the presence of these catalytic fluids permits the formation of the largest and oldest crystals.