Diamonds are crystallized carbon that survives an incredible journey from the deep Earth to the surface. Their existence is a testament to the planet’s extreme internal forces, which transform carbon into one of the hardest known natural materials. The search for these gemstones requires understanding the specific pressure and temperature conditions required for their creation deep within the Earth’s interior and locating the rare geological structures that act as conduits for their ascent.
How Diamonds Form in the Earth’s Mantle
The formation of natural diamonds requires a precise set of extreme conditions met exclusively within the Earth’s mantle. This process occurs deep beneath the continental crust, typically at depths ranging from 140 to 250 kilometers. These immense depths generate the crushing pressure required to force carbon atoms into the tightly bonded cubic crystal structure that defines a diamond.
The pressure must reach at least 4.5 to 6 gigapascals (GPa), equivalent to 45,000 to 60,000 times the atmospheric pressure at sea level. Simultaneously, the temperature must range between 900 and 1,300 degrees Celsius. This narrow pressure and temperature window is known as the diamond stability field, found only in the thick, cold roots of ancient continental landmasses called cratons.
These stable craton roots, sometimes called “keels,” extend more than 200 kilometers deep, providing the necessary environment for diamonds to remain preserved. The carbon source is varied, coming from primordial mantle material or recycled carbon carried down through subduction. Diamonds are not found where they are created; they must first be transported to the surface by a specific geological mechanism before they can be mined.
Delivery to the Surface via Volcanic Pipes
Diamonds formed deep within the mantle are brought to accessible depths through rare, high-velocity volcanic eruptions. These eruptions occur through geological structures known as kimberlite and lamproite pipes, which are the primary source of diamonds. Kimberlite is the dominant type, an igneous rock named after the South African town of Kimberley where it was first identified.
The magmas that form these pipes originate at great depths, rising rapidly through the lithosphere, often facilitated by major fault zones. This extremely fast ascent, possibly traveling at speeds of 8 to 40 miles per hour, is crucial for diamond preservation. If transported slowly, diamonds would convert back into graphite, the stable form of carbon at lower pressures, as they moved out of the diamond stability field.
As the volatile-rich magma reaches the surface, it results in an explosive eruption that creates a vertical, carrot-shaped column of rock known as a diatreme or pipe. The diamonds are not formed by the magma but are collected and carried as “accidental passengers,” becoming embedded in the cooled volcanic rock. These in-situ deposits are defined as primary sources, meaning the diamonds are still within the original host rock that delivered them to the surface.
Major Diamond Producing Regions
The distribution of diamond mines is directly tied to the presence of ancient, stable continental cratons hosting kimberlite and lamproite pipes. Russia, particularly the Siberian Craton, is one of the world’s most significant producers. Its deposits are often located in permafrost regions, containing numerous primary sources that contribute a substantial volume of the global supply.
Southern Africa is another important region, centered on the Kaapvaal Craton, which underlies countries like Botswana and South Africa. Botswana has risen to prominence as a major global source, hosting world-class mines that exploit large, consistently diamond-rich kimberlite pipes. This region’s mantle keel is exceptionally thick and cold, providing ideal conditions for long-term diamond storage.
Canada emerged as a major player in the late 20th century, with significant discoveries in the Slave Craton of the Northwest Territories. The Canadian shield’s ancient geology proved highly prospective for kimberlite pipes, establishing it as a producer of high-quality stones. Australia was home to the Argyle mine in Western Australia, a lamproite pipe that was the world’s largest producer by volume for decades and the source of nearly all pink diamonds.
Alluvial Deposits and Ancient Riverbeds
Not all diamonds are mined directly from their primary source pipes; many are found in secondary deposits called alluvial placers. Over millions of years, the kimberlite and lamproite host rocks are subjected to weathering and erosion. Since diamonds are extremely hard and chemically inert, they are liberated from the softer volcanic rock and transported by natural forces.
Water is the primary agent of this redistribution, carrying diamonds from the eroded pipes into streams and river systems. Because diamonds have a high density, they settle out of the moving water and concentrate in specific areas, such as river bends, gravel traps, or ancient riverbeds, alongside other heavy minerals. These concentrated gravels are easier to access than hard-rock pipes and were the first sources discovered historically, such as in India and Brazil.
Significant alluvial deposits are actively mined along the lower Vaal and Orange Rivers in South Africa and the coastline of Namibia, where diamonds have been concentrated by marine currents and wave action. The natural sorting process during water transport often results in a higher proportion of gem-quality stones in alluvial deposits, as flawed or weaker diamonds break apart during the journey. Mining these deposits involves processing large volumes of gravel and sand from active or ancient river channels and coastal terraces.