Diamond formation and the rock type kimberlite have existed in vague detail in my geological knowledge since my long-ago undergraduate geology classes. When the itinerary of an upcoming trip to South Africa included a possible visit to the Kimberley region, I wanted to learn more about these topics. As expected, I found fascinating information.

The Diamond Rush in South Africa

In the central region of South Africa in the 1860s, diamonds were collected on the banks of the Orange River and nearby on the hillside slopes of the De Beers farm—a location that soon became known as Kimberley. The largest of the early diamond finds included an enormous 83.5 carat specimen, sold by the shepherd who found it to a local farmer for the price of 500 sheep, 10 oxen and a horse; the principal cut of the stone is the 47.69 carat diamond known as the Star of South Africa. As knowledge of the initial discoveries spread, a diamond rush kicked off in the region, with a flood of treasure seekers racing in to seek their fortunes. Kimberley and surroundings were rapidly transformed into one of the largest diamond-producing regions in the world. (In previous centuries, diamonds from India and Brazil had spread slowly throughout the world.)

Thousands of men with picks and shovels descended on the De Beers farm hillock, rapidly lowering the land into an open pit that became the Kimberley Mine, also known as the Big Hole. From 1871 to 1914, as many as 50,000 miners dug the Big Hole. Workers excavated the pit to a depth of 790 ft (240 m), and it is 1,519 ft wide (463 m), with a surface area of 42 acres (17 h)—one of the deepest holes ever excavated by hand. The Big Hole reached depths that open-pit mining was impossible by 1885, and it was transformed into an underground mine that eventually reached a depth of about 3,600 ft (1097 m). Throughout the time of active mining, frequent accidents together with inadequate water, sanitation, and fresh food in the surrounding camps led to many deaths.

The Big Hole with the Kimberley cityscape (2023, Wikipedia)

In 1888, British businessman Cecil Rhodes founded De Beers Mining Company. The company gained control of Kimberley diamond production, as well as all other diamond mining operations in South Africa, and also established a monopoly over the world’s diamond market. Substantial profits came to De Beers from an estimated diamond yield of around 143,000,000 carats (6,173 lb; 2,800 kg), built on the unskilled labor of tens of thousands of black mineworkers. The mine owners housed workers in racially segregated compounds, and many were treated poorly during a horrific regime that was essentially slavery. The Kimberley Mine was closed in 1914—and the colonial inequalities practiced there were eventually transferred into the apartheid system.

Laborers at the Kimberley mine, 1873 (Wikimedia)

Diamonds From Deep Depths

Diamonds are a crystalline form of carbon and the hardest natural substance known. (Soft graphite is also a naturally occurring crystalline form of carbon, but in a hexagonal structure of sheets, whereas diamonds are isometric and usually occur in rounded crystals.) An igneous rock that originates at great depth within the earth, known as kimberlite, is the host rock for Kimberley diamonds and also the source of the rock name. Kimberlite is found in volcanic pipes or volcanic conduits. High concentrations of water, carbon dioxide, sulfur dioxide, and other volatile components propelled magma violently upwards toward the surface to form the rock. Relative to other igneous rock types, kimberlites exhibit low silica, high magnesium oxide and potassium oxide content, and high volatile content. These characteristics and others show kimberlites were subjected to extremely high temperatures and pressures and originated from melts deep in the mantle.

Kimberlites form at impressive depths below the surface of roughly 90 to 280 miles (150 to 450 km). Scientists estimate that during ascent, the exploding magma may hit speeds of around 125 mi/hr (200 km/hr), with transit times from the upper mantle on the order of two or three days—remarkable! Diamond-bearing kimberlite pipes are typically found in clusters that can range from several up to dozens of pipes, and they are commonly found on stable continental cratons (shields) with Archean (Proterozoic) rocks, especially in southern Africa, Canada, Siberia and northern Australia.

Outcrops of kimberlite pipes range in diameter up to about 1 mi (1.6 km), reflecting the underlying carrot-shaped structures of the pipes. The eruptions fracture rock as the magma explodes upward toward the ground surface, with only a small amount of material deposited on the surface above the buried column of magma. Over time, erosion reveals the once-buried material that holds inclusions (xenocrysts)—notably, diamonds and other gemstones, including garnets, spinels, pyrope, and peridot. These minerals and many others crystalize at depth in the magma, or are ripped from wall rocks during the ascent of the magma, or are altered from reactions with various fluids. Once exposed on the ground surface, the crystals can be transported into river gravels and other sedimentary deposits.

Diagram of volcanic pipe, showing erosion level reached by Kimberley and two other diamond mines in South Africa (2005, Wikimedia)

For earth scientists, the geochemical characteristics of kimberlites provide valuable insights into the evolutionary history and composition of our planet. The material brought to the surface from deep within the earth provides samples of the mantle that would otherwise be inaccessible.

In a search for diamonds, geophysical methods including magnetic and gravity surveys can reveal potential kimberlite targets for investigation. Scientists analyzing the mineral content of kimberlites can estimate the conditions under which the minerals formed and the likelihood of finding specific types, such as diamonds. Many diamonds that likely originated in the lower mantle appear to be derived from subducted oceanic lithosphere—a fascinating transformation, from my perspective.

Recent Events

The De Beers corporation has undergone many organizational, management, and brand changes over the decades since the 1888 founding. Effective marketing strategies successfully manipulated consumer demand—especially those advertising diamonds as a symbol of love and commitment, including the famous “A Diamond is Forever” advertising slogan. Until early in the 21st century, the organization controlled 80% to 85% of rough diamond distribution worldwide. By 2019, however, the De Beers’s market share of rough diamonds had fallen to about 30% —the consequences of an antitrust lawsuit in the United States, changes in how mine producers in Canada and Australia distributed diamonds, negative publicity around “blood diamonds”, and other factors. Today, the De Beers group continues to operate diamond mines in several parts of Africa and is involved in multiple aspects of the diamond industry, including grading and jewelry production.

In recent decades, researchers perfected the technology to produce diamonds synthetically that are visually identical to naturally occurring mined diamonds. The prices of lab-grown diamonds have fallen significantly relative to natural diamonds, and their popularity has grown. By 2024, lab-grown diamonds reportedly represented 25% of the value of the global diamond market.

My trip to South Africa was excellent, although I didn’t have the opportunity to visit Kimberley. However, I frequently observed the influence of diamonds on the history and economy of South Africa. In some ways, I think the diversity in South African society today mirrors the complex and unusual conditions under which diamonds develop. We live in an amazing world!

The nine major uncut stones split from the rough 3,106-carat Cullinan diamond found in South Africa in 1905 (circa 1908, Wikimedia)

 

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SOURCES
de Azevedo, Rafael, and Tijl Vanneste. ““The Very Soul Must Be Held in Bondage!”: Alice Victoria Kinloch’s Critical Examination of South Africa’s Diamond-Mining Compounds.” International Review of Social History 69, no. 3 (2024): 383-410.
Glazner, Allen F., and Matthew J. Kohn. Petrology and Plate Tectonics: An Earth Systems Approach. Oxford University Press, 2025, pp. 456, 460-462.
Photograph of the Big Hole with Kimberley cityscape, South Africa, 2023, by Bernard Dupont.         https://commons.wikimedia.org/wiki/File:The_Big_Hole_…_(52643915416).jpg
Photograph of Kimberley types of labor, 1873. https://commons.wikimedia.org/wiki/File:DF1873_Types_of_Labour.jpg
Diagram of volcanic pipe, showing erosion levels reached by Kimberley and other diamond mines in South Africa, 2005, by Asbestos.   https://en.wikipedia.org/wiki/Diamond#/media/File:VolcanicPipe.jpg
Photograph of the nine major uncut stones split from the rough 3,106 carat Cullinan diamond found in South Africa in 1905, circa 1908.  https://en.wikipedia.org/wiki/Cullinan_Diamond#/media/File:Cullinanroughpieces.jpg