Where Do Diamonds Come From ?

Experiments and the high density of diamonds tell us that they crystallize at very high pressures. In nature this means that diamonds are created by geologic processes at great depth within Earth, generally more than 150 kilometers down, in a region beneath the crust known as the mantle. Other processes, explored later in this exhibition, bring diamonds to where people can find them.

This diagram shows the interior structure of Earth. The three concentric layers -- the core, mantle, and crust -- formed within a few hundred million years of Earth's coalescence 4.5 billion years ago. The core is primarily an iron-nickel alloy and makes up a large fraction of the mass of Earth. The vast mantle is sandwiched between the core and the thin crust and is composed predominantly of magnesium and iron silicate minerals. Our planet's crust is a thin, rocky skin. Diamonds can form in most of Earth's interior but not near its surface, where graphite is the stable form of carbon. Indeed, diamonds only survive at Earth's surface because great heat is required to break down the diamond structure.

The upper mantle is slightly plastic, which allows it to circulate slowly in a creeping, convective flow that helps drive the surface motion of Earth known as "plate tectonics."

The cross section shown here provides a closer look at Earth's crust and underlying mantle. The crust can be divided into ocean basins, underlain by a thin layer of dense, basaltic rock, and continents, formed of a much thicker but less-dense layer of granitic rocks. Just below the crust is the portion of the mantle called the lithosphere, which is rigid and acts like rock. Below this is the asthenosphere, a more plastic, flowing region that enables the overlying crustal plates to move in what is known as plate tectonics.

The plot of pressure and temperature shows the conditions at which either diamond or graphite exist. The general conditions present in the Earth are described by curved lines called geotherms. Note that there are two geotherms: Because the continental crust is old and thick, conditions are somewhat colder in and beneath it than beneath the much younger ocean basins. Diamonds can form at depths as shallow as 150 kilometers beneath the continental crust, while beneath oceans they need depths of at least 200 kilometers, as shown by the diamond boundary on the cross-section.

Sorting and Distributing Diamonds : Form The Mine To The Diamond Dealer

There are two aspects of moving diamonds from mine to dealer. The first is the fairly straightforward but important task of separating diamonds into gem-quality, near gem-quality, and industrial-grade diamonds. The second is the more intriguing aspect: the primary diamond marketing, which has been and still is largely controlled by De Beers Consolidated Mines, Ltd. through its majority control of the Central Selling Organization (CSO). The CSO sells a large percentage of mine production to diamond dealers; independent mines sell by closed bids and through private transactions.
Sorting occurs at every level of the market, from the mine to the jeweler. At the mine, the sorting depends on the sophistication of the operation and the size of production, but it is always based on grouping stones of like type. Diamonds are grouped into "sizes" -- more than one carat; "smalls" -- between 1 carat and 1/10th carat; and "sand," -- less than 1/10th carat, with some leeway for market pressures. Diamonds larger than about 15 carats are handled individually. Shape groups comprise "stones," "shapes," "cleavages," "macles," and "flats," describing characteristics familiar to the market. The ultimate purpose of sorting is to estimate an asking price for the rough diamonds.

After great swings in diamond prices, the Diamond Trading Corporation (DTC) was set up by De Beers in 1934 to handle the actual sales of diamonds. The DTC and the Diamond Producers' Association (the mine operators) form the nucleus of the Central Selling Organization. The CSO stabilizes prices in hard times and raises them in accord with inflation and demand during good times. It needs considerable wealth and stockpiles of diamonds to maintain this position, but this "single channel marketing" system has been an effective cartel. In the United States cartels are illegal, so De Beers cannot operate here. However, the company's interests are represented by a public relations office, the Diamond Information Center, and indirectly by the diamond dealers and jewelers who sell the gems.

Alluvial Mining

Most of the diamond deposits first discovered were alluvial -- concentrations in streambed or riverbed sand and gravel. They are still actively exploited in many ways, from the most primitive to the highly sophisticated. The goal is relatively simple: to find a location where moving water has deposited diamonds in the bottom of a channel, possibly in a pocket or cleft. Because rivers meander and drainage can change, fossilizing a once active river, the search for alluvial diamonds requires some geological knowledge and a lot of luck. The process involves removing the overlying barren ground, digging up the bearing ground, extracting the diamonds, and, nowadays, restoring the landscape when finished.

In the most basic, individual operations, such as in Sierra Leone or Angola, the technology involves shovel and pan, with some hand sloshing to gravitate diamond to the bottom of the pan; the eye is the ultimate sorting device. Mom-and-pop operations in South Africa involve a small claim and utilize limited technology -- shovels, buckets, jury-rigged cranes powered by small vehicles, and the like -- to load a small washing pan. The concentrate is then sieved into several size ranges, and each fraction is dumped onto a picking table, where someone checks by eye for diamonds. In the bigger operations, as shown in the model, large earth-moving equipment transports the alluvium, and the processing approaches that of the primary mines -- coarse sieving, then rotary sieving in a trommel, before loading into a large washing pan. Final processing includes concentrate sieving, a picking table, and usually a grease table. Of course, no crushing is required, as nature has already released the diamonds from the pipe rock.

Mining Marine Deposits

Marine deposits are a variation on alluvial deposits. They result from the wave action of the ocean, which has concentrated diamonds at the base of the surf zone. Waves arriving at an angle to the coast tend to push the diamonds along the coast, causing the diamonds to stream out from where rivers deposited them at the coastline. Moreover, changes in climate have led to great variations in sea level -- hence movement of diamond concentrations to both old beaches well up on land and others now more than 100 meters below sea level.

The mining vessel "Geomaster" of the De Beers Marine fleet working off the coasts of Namibia and Namaqualand, South Africa. It is capable of operating in depths up to 200 m (660 feet).
There are 3 types of marine mining operations. In one, sand is moved from 10 meters below sea level to as far inland as the sea may have risen, in order to reveal the concentrations on the bedrock. In another, divers and boats work in the surf zone to perhaps 20 meters of water and use suction pipes to remove gravel and diamonds from the ocean floor. In the third, deep-sea marine vessels use remote underwater tractors or large underwater excavators to remove overlying sediments and extract the diamond-bearing sand and gravel. Processing is done on land in the first 2 cases and shipboard on the large mining vessels.

Wave refraction maps like this one for concession 2(b) along the South African coast are used to assist in locating concentrations of diamonds. The map shows where the wave force is maximized, and thus where diamonds will have been concentrated by wave action. Wave power is transformed into the visible spectrum, so that blue represents weak and red strong power. click to zoom in

Processing Diamond Ore

Once a mining operation yields ore, the diamonds must be sorted from the other materials. This process relies primarily on diamond's high density. An old but effective method is to use a washing pan, which forces heavy minerals like diamond to the bottom and waste to the top. Cones and cyclones use swirling heavy fluids mixed with crushed ore to achieve density separations. With 99 percent of the waste in the ore removed, further separations may use either a grease table or an x-ray separator. Final separation and sorting is done by eye.

Crushed ore is mixed with a muddy water suspension, called puddle, and all is stirred by angled rotating blades in the circular washing pan. Heavier minerals settle to the bottom and are pushed toward an exit point, while lighter waste rises to the top and overflows as a separate stream of material.

A working grease table at Longlands, Cape Province, South Africa. The surface of diamond is highly unusual in that it resists being wetted by water but sticks readily to grease. Here, wet gravel washes across 3 inclined surfaces covered with beeswax and paraffin. Diamonds stick to the grease while wetted waste minerals flow past. The operator routinely scrapes the material that adheres to the table into a grease pot, using a trowel. The grease in the pot is melted and the diamonds are removed in a strainer. More automated systems use a rotating grease belt and scraper.

This diagram shows how cones (left) and cyclones (right) use heavy-media separation. Diamond-bearing concentrate is mixed with a fluid near the density of diamond. Separation occurs in cones and cyclones by swirling the mixture at low and high velocities respectively. In the cone, rotational mixing permits lighter minerals to float to the top and run out as overflow, while diamonds and dense minerals sink to the bottom and are sucked out with a compressed air siphon. In the cyclone, fast rotation of the suspension drives heavy minerals to the conical wall, where they sink to the bottom and are extracted, while float waste minerals are sucked from the center of the vortex. Cyclones are about 99.999% efficient at concentrating diamonds and similarly dense minerals from the original ore. Adapted from Bruton (1978)

The x-ray separator system acts on a thin stream of particles from the concentrate accelerated off a moving belt into the air, where they encounter an intense beam of x-rays. Any diamond fluoresces in the x-rays, activating a photomultiplier that triggers a jet of air, deflecting the diamonds (red) into a collector bin. Adapted from Bruton (1978)

Aining a Kimberlite Pipe

Mining of a diamond-bearing pipe starts with the excavation of a pit into the pipe. In this process, called "open-pit" or "open-cast" mining, the initially loose and eventually hard ore material is removed with large hydraulic shovels and ore trucks. Hard rock is drilled and blasted with explosives so the broken material can be removed. When deep, rich ore warrants it, the mining goes underground with vertical shafts descending to horizontal drifts, or passageways that enter the pipe.

In bedrock adjacent to the pipe, shafts are sunk and drifts are tunneled into the pipe. The highly mechanized and efficient method known as block caving is shown in the adjacent model. Concrete-lined tunnels are excavated under a large vertical section, perhaps 140 to 180 meters (400 to 600 feet) of kimberlite. Along the tunnels are draw points, or openings in the concrete casing where kimberlite is The open pit at the Udachnaya (Lucky) pipe in Sakha Republic, Russia, has produced many millions of carats of diamond. The bench steps are about 20 m high.drilled and blasted to cave in a section above the tunnel. Broken kimberlite falls through the draw points and is scraped out of the tunnel with a drag or scraper bucket attached to a cable and winch, working much like a clothes line on a pulley. The kimberlite above the tunnels falls under its own weight and leads to a slow, continuous caving of ground that is removed through the draw points. The scraped kimberlite rubble is loaded into cars on a lower level and moved to a crusher underground. The crushed ore is then conveyed to skips that carry the ore up the vertical shaft for processing.

The Types Of Diamond Deposits

Geologic processes create two basic types of diamond deposits, referred to as primary and secondary sources. Primary sources are the kimberlite and lamproite pipes that raise diamonds from Earth's mantle, where they originate. Secondary sources, created by erosion, include such deposits as surface scatterings around a pipe, concentrations in river channels, and fluxes from rivers moved by wave action along ocean coasts, past and present. Mining of these deposits depends upon sufficient concentration and quality of diamonds.

This diagram shows the trail of diamonds left by geological processes. The primary deposits, or diamond pipes, are the vertical portion. The flared top of the pipes can yield substantial quantities of diamonds, but following the narrowing pipe downward eventually becomes unprofitable. Note how erosion of the landscape moves surface minerals -- including the diamonds -- from the pipes down hills, streams, and rivers to their ultimate destination, the ocean. Because diamonds are dense they concentrate at the bottom of active zones of moving sand and gravel. These secondary deposits are eluvial &#eth; above a pipe, colluvial -- adjacent to a pipe, alluvial &#eth; stream and river transported, and marine -- along beaches that can wind up onshore or offshore with changing sea level. Secondary deposits may be found far from active means of transport, in the fossilized channels of now-vanished rivers or under fossil beaches.

North America Diamonds

UNITED STATES: Although numerous kimberlites and lamproites are scattered from New York to Wyoming and Michigan to Arkansas, diamond discoveries and mining have been minimal in the United States, restricted to Arkansas and, now, Colorado.
Near Murfreesboro, Arkansas, from what is now known to be a lamproite pipe, more than 100,000 stones averaging 0.25 carat each were produced between 1907 and 1933. Now Crater of Diamonds State Park, the locality is a stop for hobbyists and tourists. The "State Line Kimberlite District" along the Colorado-Wyoming border is home to the only operating diamond mine in the U.S., started at Kelsey Lake, Colorado, in June 1996. The largest diamond found so far is 28.3 carats, and about 25% of the 20,000-carat annual production is of gem quality.

CANADA: With much of Canada underlain by ancient bedrock, the existence of diamond-bearing kimberlite has been considered very likely, particularly with tantalizing diamonds found in Wisconsin in deposits swept down by glaciers from up north. Now the country is home to the latest major diamond discovery.

Intermittent exploration for kimberlites by major companies was unsuccessful through the 1980s. But Chuck Fipke, head of Dia Met Minerals, persevered and in April 1990 located a kimberlite under Point Lake. Eventually, his company, with partner BHP Minerals, found more than 100 kimberlites on their claims; 42 contain diamonds. Five small pipes, to be operated as if they were one mine, are scheduled to start production in the second half of 1998.

Australia Diamonds

AUSTRALIA: While minor diamond discoveries were made among alluvial gold in New South Wales starting in 1851, a discovery in 1979 on the Kimberley Plateau of Western Australia enabled the country to be the world's most prolific diamond producer.


Based on ancient bedrock, diamond exploration began in 1972, with a kimberlite pipe discovery coming in 1976 in the Ellendale area. In 1979, a large lamproite pipe was found and named the Argyle mine; by 1992 over 200 million carats had been mined there. Only 5% of the production is gem quality. A unique feature of the Argyle mine, though, is a small but consistent supply of valuable pink to red or purple diamonds.

Australian Production:

Total: 428 million carats

Annual: 35-40 million carats

Russia Diamonds

The greatest success story of diamond exploration in the 20th Century is Russia. In the 1930s, academician Vladimir S. Sobolev recognized similarities between the ancient bedrock of Siberia and that of South Africa and Botswana. He concluded that major diamond deposits in Siberia were possible.

In 1947, prospecting started by looking in streambeds for trails of indicator minerals such as pyrope garnet and ilmenite, which point to primary diamond deposits. Within a decade the efforts succeeded, in spite of very difficult Arctic wilderness conditions. In 1954 in Sakha (formerly Yakutia), the first kimberlite pipe, Zarnista (Dawn) was found in the Daldyn field; in 1955, the very rich Mir (Peace) and Udachnaya (Lucky) pipes were discovered within 10 days of each other but about 400 km (250 miles) apart. By 1956, more than 500 kimberlites had been discovered. In a harsh and remote area, deposits were developed gradually from 1957, and by 1970 Russia had become the third largest producer. It is now the fourth largest producer on the basis of weight. Its production is about one quarter gem diamonds, worth $1.2 billion, second to Botswana.

The Internationalnaya mine, only 40 km from the Mir (Peace) pipe, has terminated above ground operations, and underground operations have been hampered by waters that emit toxic gases.

The Mir mine in the Malaya Botuobiya field was the first Russian mine opened in 1957. Annual production has averaged about 2 million carats. Although the pipe has been mined out, stockpiles and tailings (waste) continue to be worked for diamonds.

Blasting to dislodge hard kimberlite at the Udachnaya mine, the most productive one in the Sakha Republic.

Russian Production:

Total: 332 million carats

Annual in 1970: 7.5 million carats

Today: 10-12.5 million carats

India Diamonds

Diamonds were discovered in India by the 4th century BCE. In addition to the diamond legends, India yielded many legendary diamonds, including the Koh-i-Noor, the Orlov, the Hope, and the Sancy. Except for a minor supply of diamonds from the Kalamantian deposits of Borneo, dating from the 6th century CE, India was the world's only source until the 1730s. Most of India's deposits were alluvial, but today the Majhgawan pipe, a primary source near Panna, is the country's only producing diamond source.

Indian Production:

Total: 21 million carats

Maximum annual: 50,000 to 100,000 carats in late 1600s

Today: 20,000 carats

Brazil and Venezuela

When diamonds were discovered by alluvial gold miners in Brazil in 1725, Indian diamond sources were near exhaustion and European demand for the stone continued unabated. From 1730 to 1870 Brazil was the world's major source of diamonds. Indeed, mining in Brazil was so active that by the late 1730s production far exceeded demand, and diamond prices fell by as much as 70%. Beginning in 1850, production rose again, following the discovery of rich deposits in Bahia, but after 1861 it rapidly declined as deposits were depleted, leading to a great shortage of rough diamonds in the European cutting centers in the late 1860s.

Productive Brazilian diamond deposits are all secondary. They are usually small, and some are of low grade, so mines typically operate for short periods. Primary diamond pipes exist but are generally uneconomic, suggesting that erosion has stripped away the richest portions from them. In 1890 and 1901, secondary diamond deposits were discovered in Guyana and eastern Venezuela, adjacent to deposits in Brazil's northern state of Rora’ma. Since 1890, Guyana and Venezuela have produced a total of about 4.5 and 14 million carats, respectively.

Total: 55 million carats

Old annual: 50,000 to 300,000 carats; 1730s to 1861

Today: 1.5 million carats

Other Countries Of Africa

Africa is the richest continent for diamond mining, accounting for roughly 49% of world production. The major sources are in the south with lesser concentrations in the west-central part of the continent. The major producing countries are Congo Republic (Zaire), Botswana, South Africa, Angola, Namibia, Ghana, Central African Republic, Guinea, Sierra Leone, and Zimbabwe. Political turmoil in some countries has led to highly variable production and severe degradation of the environment from uncontrolled mining.
Congo Republic (Zaire): With 18% of world production, this is the second largest producer of diamonds by weight (20 million carats in 1995) after Australia. Only 6% is of high gem quality; another 40% consists of small stones, called near-gem, that are cut in India. Mbuji-Mayi is one of the world's most prolific mines. In recent years production has been about 5 million carats per year.

Botswana: This is the third largest producer of diamonds. De Beers discovered 3 world-class kimberlite deposits in Botswana between 1967 and 1973. The Orapa pipe, found in 1967, is the second largest economic kimberlite pipe in the world and produces about 5.5 million carats annually. The Jwaneng pipe was discovered in 1973 under the sands of the Kalahari Desert. With over 10 million carats produced in 1995, it was the second most productive mine for weight of diamonds but first for currency value. Two smaller pipes, Letlhakane 1 and 2, were discovered in 1968. Botswana's diamond reserves are immense.

Namibia: In 1908 a railroad worker found diamonds in the sand dunes near Kolmanskop, South West Africa (now Namibia), then a German colony. In 1909 almost 500,000 carats were produced there, and yields almost tripled in 5 years. The diamonds were small but of high quality. South Africa gained control of Namibia after World War I and sold the diamond deposits to Consolidated Diamond Mines (CDM), which was transferred to De Beers in 1929. In 1994 CDM was reconstituted as Namdeb Diamond Corporation Limited and is now jointly owned by the Namibian government and De Beers. The production, 1,300,000 carats in 1995, is from beach deposits.Other significant African producers and their percentage of world production in 1996:
Angola 1.8%
Ghana 0.7%
Central African Republic 0.6%
Guinea 0.5%
Sierra Leone 0.3%
Zimbabwe 0.2%

South Africa

The 1867 discovery of diamonds in the Cape Colony, now a province in South Africa, radically modified not only the world's supply of diamonds but also its conception of them. As annual world diamond production increased more than tenfold in the following 10 years, a once extremely rare material became more accessible to Western society with its growing wealth, science learned that diamonds came from volcanoes, and everyone learned of Cecil John Rhodes, Barney Barnato, Kimberley, and De Beers. Today South Africa maintains its position as a major diamond producer.

The story of diamonds in South Africa begins between December 1866 and February 1867, when 15-year-old Erasmus Jacobs found a transparent stone on his father's farm, on the south bank of the Orange River. Over the next 15 years, South Africa yielded more diamonds than India had in over 2,000 years. This great outpouring of diamonds coincided with depletion of Brazilian deposits and with a great rise in wealth, particularly in the United States, that ensured diamond prices did not fall as they did when Brazil outproduced diamond demand in the 1730s.

Plan of concessions and claims at the Kimberley mine from June 30, 1883. click to zoom inThe first diamond discoveries in South Africa were alluvial. By 1869, diamonds were found far from any stream or river, first in yellow earth and below in hard rock called blueground, later called kimberlite, after the mining town of Kimberley.

In the 1870s and 1880s Kimberley, encompassing the mines that produced 95% of the world's diamonds, was home to great wealth and fierce rivalries, most notably that between Rhodes and Barnato, English immigrants who consolidated early 31-foot-square prospects into ever larger holdings and mining companies. In 1888, Rhodes prevailed and merged the holdings of both men into De Beers Consolidated Mines Ltd., a company that is still synonymous with diamonds. Today South Africa is third in production in terms of value and is likely to stay that way for the foreseeable future.

The Kimberley mine in 1873. Visible are the roadway and the square claims which were 31 feet on a side, creating a terrain of holes and walls that made mining difficult and hazardous. From "The Diamond Mines of South Africa," G.F. Williams, New York, 1906. click to zoom in

Barney Barnato arrived in South Africa from England in 1873 at the age of 20. In 1876 he bought four claims in the Kimberley mine. He made a huge profit and later formed the Barnato Diamond Mining Co., which he merged with Kimberley Central Mining Co. in 1883. In competition, Rhodes sold one of his companies to Barnato's Kimberley Central but his retained interests gave Rhodes a 20% share in Kimberley Central. Rhodes and Barnato battled viciously for the remaining stock. In 1889, Barnato sold out to Rhodes for £5,338,650, at that time $25,000,000, paid with the largest check ever issued at that time. In 1896, Barnato disappeared at sea while on passage back to England, a presumed suicide.

In 1870, at the age of 17, Cecil John Rhodes followed his brother to South Africa. There he purchased diamond claims and became prosperous pumping water from the deepening, flooded claims in Kimberley. After expansion of his holdings and competition with Barney Barnato, Rhodes went on to form De Beers Consolidated Mines. He became prime minister (some say dictator) of the Cape Colony, and dedicated his final years to the creation of Rhodesia (now Zimbabwe). In 1902 at age 49, Rhodes died a very controversial figure. From "The Diamond Mines of South Africa," G.F. Williams, New York, 1906.

South African Production:

Total: 505 million carats

Annual 1870: 269,000 carats

1903: 3 million carats

Today: 8 to 10 million carats

Diamonds Across Southern Africa

Diamonds have been transported across vast area of southern Africa. There are more than 3,000 kimberlite pipes -- many not bearing diamonds, of course -- in the craton drained by tributaries to the Orange River and its precursors, which end at the Atlantic coast. The rivers carried water, sediments, and diamonds to the ocean.

Over the past 100 million years up to 1,400 meters have been eroded from the land's surface, releasing billions of carats of diamonds on a trip to the sea. An estimate of diamonds eroded from the Kimberley mine -- the "Big Hole" -- alone is 500,000 carats. The rivers carried most of the eroded diamonds to the Atlantic Ocean, although about 10% of them were store in the alluvial deposits of the drainage systems. The rest are in the past and present beach deposits of the Atlantic coast, from Port Nolloth in Namaqualand to Luderitz in Namibia.
Because powerful ocean waves break the poorer quality diamonds, 90-95 percent of marine diamonds are of gem quality. The littoral zone, the area of wave action on the Atlantic coast where diamonds accumulate, has moved in and out with changes in sea level, but shore lines have been constant over long periods, resulting in wave-cut terraces with hollows and crevices in which diamonds concentrated. These terraces are preserved hundreds of meters both above and below sea level and are the focus of mining activity.

Today diamonds are mined in about 25 countries, on every continent but Europe and Antarctica. However, only a few diamond deposits were known until the 20th century, when scientific understanding and technology extended diamond exploration and mining around the globe. For 1,000 years, starting in roughly the 4th century BCE, India was the only source of diamonds. In 1725, important sources were discovered in Brazil, and in the 1870s major finds in South Africa marked a dramatic increase in the diamond supply. Additional major producers now include several African countries, Siberian Russia, and Australia.
It is a modern misconception that the world's diamonds come primarily from South Africa: diamonds are a world-wide resource. The common characteristic of primary diamond deposits is the ancient terrain that hosts the kimberlite and lamproite pipes that bring diamonds to Earth's surface

The map above shows both the major deposits and the ancient bedrock, both the 2,500-million-year-old archons and less productive 1,600 to 2,500-million-year-old protons, that contain the diamond pipes. The diamonds in secondary deposits have been moved by erosion away from the pipes.

The monumental increase in diamond production in the 20th century is shown on this graph. India's maximum production, perhaps 50,000 to 100,000 carats annually in the 16th century, is very small by modern standards. Brazil and Venezuela are barely discernible compared to South African production following discoveries in 1867. For the most part, except for major wars and economic recessions, diamond production has been steadily increasing since then, with non-African sources growing in relative proportion. Major production is now dominated by Australia, Botswana, Russia, and Congo Republic (Zaire), but South Africa is still a major producer, in both volume and value.