The belief that diamonds form from coal is a common misconception. While both are primarily carbon, direct transformation of coal into diamonds does not occur naturally or synthetically.
Natural Diamond Formation
Natural diamonds originate deep within the Earth’s mantle, at depths typically ranging from 140 to 190 kilometers (approximately 90 to 120 miles) below the surface. These extreme conditions are necessary for carbon atoms to arrange into the rigid, crystalline structure that defines a diamond. Temperatures in this region generally range from 900 to 1,300 degrees Celsius, coupled with pressures of 4.5 to 6 gigapascals, which is roughly 45,000 to 60,000 times atmospheric pressure.
The carbon source for these deep-Earth diamonds is not coal, but ancient carbon trapped within the mantle or carbon introduced through oceanic plate subduction. Under this intense heat and pressure, carbon atoms bond to form diamonds’ dense, tetrahedral arrangement. Once formed, these diamonds are brought closer to the Earth’s surface through specific volcanic eruptions, transported rapidly by molten rock (kimberlite and lamproite) through vertical pipes.
The Composition of Coal
Coal is a sedimentary rock formed from ancient plant matter that has undergone millions of years of burial, heat, and pressure. This process, known as coalification, transforms decaying vegetation into a carbon-rich material. Coal is primarily carbon, with its content varying from 65% to 95% depending on its type or rank.
Despite its high carbon content, coal also contains significant amounts of other elements and impurities, including hydrogen, oxygen, nitrogen, sulfur, and various minerals. Unlike diamonds’ highly ordered crystalline structure, coal has an amorphous structure, meaning its carbon atoms lack a regular, repeating pattern. This atomic arrangement and the presence of impurities distinguish coal from the pure carbon sources needed for diamond formation.
Creating Diamonds in a Lab
Humans have successfully replicated diamond formation in controlled laboratory environments using two primary methods: High Pressure, High Temperature (HPHT) and Chemical Vapor Deposition (CVD). The HPHT method mimics the natural conditions found deep within the Earth. It involves subjecting a pure carbon source, such as graphite, to pressures of 5-6 GPa and temperatures between 1,300 and 1,600 degrees Celsius. A metal catalyst often facilitates the dissolution of carbon, allowing it to crystallize around a small diamond seed.
The CVD method involves a vacuum chamber filled with carbon-containing gases, typically methane and hydrogen. Energy, often from microwaves, creates a plasma that breaks down the gas molecules. Carbon atoms then deposit layer by layer onto a seed crystal, gradually building the diamond structure. Lab-grown diamonds use purified carbon sources, not coal, and are created under precisely controlled conditions.
Why Coal Doesn’t Become Diamond
Despite both being carbon-based, key differences in their origin, formation conditions, and composition prevent coal from transforming into diamonds. Natural diamonds form from ancient carbon sources deep within the Earth’s mantle, predating most coal deposits. Coal, conversely, originates from plant matter that accumulated much closer to the Earth’s surface.
The geological conditions for coal formation involve lower pressures and temperatures compared to those required for diamonds. Coal forms under conditions that lead to its amorphous structure and the retention of various impurities. Diamonds, whether natural or lab-grown, require high pressures and temperatures to form a highly ordered crystalline lattice, a process incompatible with coal’s impure and disordered composition.