Diamonds are among the most durable and prized materials on Earth, formed under conditions of extreme pressure and temperature far beneath the surface. This unique combination of subterranean forces transforms simple carbon atoms into dense, sparkling crystals. The journey of a diamond from its deep-Earth birthplace to the surface involves explosive, high-speed geology.
The Crystalline Nature of Diamonds
A diamond is chemically pure carbon, but its extraordinary properties stem entirely from the arrangement of its atoms. Each carbon atom is covalently bonded to four neighbors, forming a rigid, three-dimensional tetrahedral lattice. This dense, tightly packed atomic structure makes diamond the hardest natural material known.
Graphite, the carbon allotrope seen in pencils, has a vastly different atomic arrangement. Graphite atoms bond in flat, layered hexagonal sheets that are weakly connected, allowing them to slide easily. The transition from graphite’s loose structure to diamond’s dense structure requires immense pressure to force the atoms into the compact configuration.
The Earth Layer Where Formation Occurs
Most diamonds crystallize in the “diamond stability field,” a specific pressure-temperature regime deep within the Earth’s mantle. The majority of gem-quality diamonds form at depths ranging from approximately 140 to 250 kilometers below the continental surface. This zone is part of the lithospheric mantle, the solid, uppermost layer of the mantle attached to the base of the crust.
Diamond formation requires precise and extreme conditions, with temperatures between 900°C and 1,300°C. The pressure must be crushing, typically ranging from 45 to 60 kilobars (4.5 to 6.0 gigapascals). These conditions are primarily found beneath the stable roots of ancient continents, called cratons, which have thick lithospheric keels. This stability allows the carbon to remain in its diamond form for millions or even billions of years.
Carbon sources include primordial mantle carbon and carbon recycled from the Earth’s surface through subduction zones. The carbon atoms precipitate out of carbon-containing fluids or melts interacting with the surrounding mantle rock. The process of crystallization is slow, with the oldest diamonds having formed over three billion years ago.
The Journey to the Surface
Diamonds formed in the deep mantle are only available for mining due to a rare and violent form of volcanism. They are brought to the surface by explosive eruptions of magmas originating at the same great depths. These eruptions create narrow, carrot-shaped geological structures known as kimberlite pipes, and less commonly, lamproite pipes.
The rapid ascent of the magma is necessary to preserve the diamonds. If transport were slow, the diamonds would pass through a lower pressure zone where graphite is stable, causing them to convert back into graphite. The volatile-rich kimberlite magma acts like a high-speed elevator, blasting through the crust.
The magma ascends at speeds that prevent the diamond from dissolving or transforming into graphite as it nears the surface. The resulting kimberlite pipes contain diamonds carried intact from the mantle in the fast-moving volcanic material. These pipes are the primary source of natural, mined diamonds today.