Diamonds are naturally occurring minerals composed almost entirely of pure carbon. Their exceptional hardness and brilliance have long captivated humanity, making them highly valued. This allure is further intensified by their rarity and the extreme conditions necessary for their formation deep within the Earth.
How Diamonds Form
Diamonds originate deep within the Earth’s mantle, at depths ranging from 150 to 200 kilometers (90 to 125 miles) below the surface. Some rare diamonds, however, can form at even greater depths, up to 800 kilometers (500 miles). These environments are characterized by immense pressure, ranging from 45,000 to 60,000 atmospheres, and high temperatures between 900 and 1,300 degrees Celsius (1,652 to 2,372 degrees Fahrenheit).
The carbon source for diamond formation comes from two main origins. It can be primordial carbon, present in the mantle since Earth’s early stages, or carbon introduced into the mantle through the subduction of oceanic crust. During subduction, oceanic plates dive beneath continental plates, carrying carbon-bearing minerals deep into the Earth’s interior. Under these conditions, carbon atoms crystallize into the cubic lattice structure that defines a diamond.
Once formed, these diamonds are brought closer to the Earth’s surface through a geological process. Deep-seated volcanic eruptions rapidly transport diamond-bearing rock from the mantle. This swift ascent prevents the diamonds from reverting to graphite, which is the stable form of carbon at lower pressures and temperatures.
Primary Diamond Sources
The geological structures that directly transport diamonds from the Earth’s mantle to the surface are known as primary sources. The most significant are kimberlite pipes, carrot-shaped volcanic conduits. These pipes formed from deep-seated volcanic eruptions that forced molten rock and diamonds upwards through the Earth’s crust. The magma cooled and solidified within these vertical structures, embedding the diamonds within a rock known as kimberlite.
Lamproite pipes represent another type of primary diamond source. These pipes are considered “in-situ” sources because the diamonds are found in their original host rock. Only a small fraction contain economically viable quantities of gem-quality diamonds.
Secondary Diamond Sources
Diamonds can also be found in locations away from their original primary sources. These deposits form through the natural processes of erosion and transportation. Over millions of years, primary kimberlite and lamproite pipes are subjected to weathering, which breaks down the surrounding rock. Because diamonds are exceptionally hard, they resist this erosion and are released from their host rock.
Once liberated, diamonds are transported by water. Rivers are a primary agent, carrying diamonds downstream and depositing them in riverbeds, floodplains, and deltas, forming alluvial deposits. Ocean currents also move diamonds along coastlines and deposit them in marine environments, both nearshore and offshore. These secondary deposits are often more accessible and can be mined using different techniques compared to deep, hard-rock primary pipes.
Global Diamond-Producing Regions
Diamond production occurs across several continents. Russia holds the position as the world’s leading producer of gem-quality diamonds by carat weight, primarily from deposits in Siberia. Their operations include numerous open-pit mines.
Africa is a major region for diamond mining, with Botswana being a major producer by value due to the high quality of its diamonds. Other African diamond-producing countries include South Africa, Angola, and the Democratic Republic of Congo. Canada has emerged as a significant producer in recent decades, and Australia also produces diamonds. These regions host a combination of primary kimberlite and lamproite deposits, as well as extensive secondary alluvial and marine diamond fields.