Diamond is a solid form of the element carbon, characterized by its atoms arranged in an extremely rigid, three-dimensional crystal structure known as diamond cubic. This unique structure results in the material’s unparalleled hardness and exceptional optical properties. The formation process requires specific, rare conditions deep within the planet, which is the primary reason for its rarity and high value.
The Necessary Ingredients and Conditions
The transformation of carbon into diamond is governed by a precise combination of immense heat and pressure. Carbon atoms must be forced into the dense tetrahedral lattice structure, a state only stable under extreme geophysical conditions. The required temperature range typically falls between 900 and 1,300 degrees Celsius, providing the energy necessary for the carbon atoms to rearrange themselves.
This high temperature must be paired with enormous pressure, usually between 45 and 60 kilobars (approximately 50,000 times the atmospheric pressure). At lower pressures, carbon naturally crystallizes as graphite, a much softer material. The carbon source can come from deep primordial reservoirs within the mantle or from subducted organic material. These specific conditions restrict natural diamond formation to particular geological zones far below the Earth’s crust.
Deep Earth Mantle Formation
The vast majority of natural diamonds are formed within the Earth’s mantle, at depths ranging from about 140 to 250 kilometers. This formation zone is located within the lithospheric mantle beneath the oldest and most stable continental interiors, known as cratons. These ancient continental roots have thick, cool mantle keels where the high-pressure, high-temperature environment remains stable for billions of years.
The slow, steady conditions within these cratonic keels allow crystallization to occur over immense timescales, sometimes taking millions or even billions of years. Carbon is introduced through two primary sources, defined by the host rock: peridotitic (typical upper mantle rock) and eclogitic (originating from subducted oceanic crust).
Diamond formation often involves carbon-bearing fluids and melts migrating through the mantle rock, dissolving carbon and precipitating it as diamond when chemical conditions are right. For instance, the reduction of carbonate minerals or the oxidation of methane-rich fluids can cause precipitation. Diamonds formed here are the oldest and most common type, providing preserved samples of the deep Earth’s ancient history.
Transport to the Surface
Once diamonds are formed, they must be carried to the surface very rapidly to prevent them from reverting into graphite under lower-pressure conditions. This upward transport is achieved by rare, violent volcanic events that originate deep within the mantle. These eruptions are thought to be some of the most explosive on Earth, acting as a high-speed elevator for the deep-seated rocks.
The volcanic material that hosts the diamonds is primarily a rock known as kimberlite, and less commonly, lamproite. This magma punches through the continental crust, creating vertical, carrot-shaped geological structures called kimberlite or lamproite pipes. The speed of the ascent is critical, as it flash-freezes the diamonds into the rock before the pressure and temperature drop, preventing instability.
The presence of volatile components like carbon dioxide and water in the magma helps facilitate this rapid, explosive journey. Without this forceful delivery system, the diamonds would never survive the trip through the upper mantle and crust to be accessible for mining.
Alternative Formation Methods
While the deep mantle is the source of all gem-quality diamonds, two other processes account for diamond formation in different geological settings. One alternative involves shock metamorphism during high-energy impact events, such as a large meteorite striking the Earth. The instantaneous, extreme pressure and temperature generated by the impact convert carbon in the target rock or the meteorite into microscopic diamonds. These tiny crystals, sometimes called nanodiamonds, are too small for jewelry but provide a geological record of the cosmic impact.
Another method is the laboratory creation of synthetic diamonds, primarily through two techniques. The High-Pressure/High-Temperature (HPHT) method directly mimics the mantle environment by subjecting a carbon source to pressures of about 5–6 GPa and temperatures exceeding 1,300°C. Conversely, the Chemical Vapor Deposition (CVD) process uses a vacuum chamber where carbon-rich gases are broken down into plasma, allowing carbon atoms to deposit layer-by-layer onto a diamond seed crystal at much lower pressures. These synthetic methods produce diamonds chemically identical to natural stones, serving both industrial and jewelry markets.