Diamonds are a unique form of carbon, distinct from graphite, the more chemically stable form of the element at the Earth’s surface. As the hardest known natural substance, diamonds possess a crystalline structure that forms only under extreme geological conditions. The extraordinary pressures and high temperatures required for this transformation mean their natural occurrence is exceptionally rare. The journey of a diamond from its birth beneath the Earth to its final location for discovery is a complex geological saga. This inherent scarcity, dictated by the planet’s internal dynamics, makes them a prized resource.
How Diamonds Form in the Earth’s Mantle
The genesis of natural diamonds is confined to a specific zone deep within the Earth’s interior, known as the diamond stability field. This region exists exclusively in the upper mantle, at depths ranging from 150 to 250 kilometers beneath the surface, where carbon-bearing materials are subjected to immense geothermal forces.
Diamond crystallization requires temperatures between 900 and 1,300 degrees Celsius, paired with pressures of 45 to 60 kilobars. These extreme parameters are only sustained beneath the thickest, oldest sections of the continental crust, geological structures called cratons. The mantle material beneath these cratons acts as a long-term storage location where diamonds remain preserved until a geological event brings them closer to the surface.
Primary Sources Kimberlite and Lamproite Pipes
Diamonds do not form near the surface, so their discovery depends on a rapid, violent transport mechanism from the mantle. This transport is facilitated by deep-seated volcanic eruptions that occur when unique magmas force their way upward through fractures in the cratons. These eruptions are fast, preventing the diamonds from reverting back to graphite under lower pressure conditions.
The diamond-bearing magma solidifies into vertical, carrot-shaped geological structures known as pipes. The vast majority of mined diamonds originate from two specific types of igneous rock within these conduits: kimberlite and, less commonly, lamproite. These primary sources represent the original location where the diamonds were deposited after their rapid ascent from the mantle.
Secondary Sources Alluvial and Marine Deposits
Not all diamonds are mined directly from their original volcanic pipes; many are found in secondary locations far from their source. These secondary deposits, known as alluvial or placer deposits, are formed through the long-term process of weathering and erosion. Over millions of years, the softer host rock is broken down by natural forces.
Once freed from the pipe, diamonds are transported by rivers and streams and deposited in new sedimentary environments. Because diamonds are extremely dense and chemically resistant, they concentrate in riverbeds, floodplains, or ancient river channels alongside other heavy minerals. This natural sorting often results in alluvial deposits yielding a higher percentage of gem-quality stones, as weaker diamonds are destroyed during transport. Marine deposits are alluvial diamonds further transported and concentrated by ocean currents and wave action along coastlines.
Geography of Current Diamond Production
The world’s diamond supply is dominated by key geographical regions situated atop ancient cratons. Russia, primarily in the Siberian region of Yakutia, is the world’s largest producer by volume, relying on primary kimberlite pipes such as the Udachnaya and Aykhal mines.
Botswana often leads the world in production value due to the exceptional quality of its stones, sourced from prolific kimberlite deposits like the Jwaneng and Orapa mines. Canada has also emerged as a major producer, with mines like Diavik and Ekati located in the remote Northwest Territories.
While Australia’s historic Argyle mine, a lamproite deposit famous for its rare pink diamonds, has closed, it remains a significant example of a primary source outside of kimberlite. Coastal regions in Southern Africa, particularly Namibia and parts of South Africa, yield significant quantities of high-quality diamonds from marine and alluvial deposits. The Democratic Republic of Congo also produces a high volume of diamonds, though the majority are lower-value stones recovered primarily through alluvial mining operations.