While trekking South Africa in 2023, I found myself exploring the Big Hole, Kimberly, 480 km south-west of Johannesburg, (pictured below).

in 1871, a member of Fleetwood Rawstone’s ‘red Cap Party’ found an 85-carat diamond on the small hill that was known by diamond diggers as Colesberg Kopje. This discovery sparked the biggest diamond rush the world has ever seen, an event that was to change South Africa’a fortunes forever. The hill was excavated and became the famous Kimberley Mine, one of five large diamond mines in the area operated by De Beers Consolidated, and once the richest diamond field in the world. Originally known as New Rush, the town that rapidly established was renamed Kimberly in 1973 after Lord Kimberly, then Secretary of State for the African Colonies.

The discovery of diamonds at Colesberg Kipje on the De Beer brothers’ property led to a crazy scramble for claims and within day 800 claims had been staked; soon more than 2,000 hopeful prospectors were at work. Eventually an open pit formed, known as the Kimberley Mine and now the ‘Big Hole’.

Picture above, kimberlite was reached at a depth of around 20-30m and was soon realized that the diamonds came from the kimberlite pipe that continued downwards.

Pictured above, the Kimberley Pipe is one of several diamond-rich volcanic pipes and smaller intrusions that cluster around Kimberley. This was a volcanic event that took place during the mid-Cretaceous. The volcanic pipe intruded explosively through the Ventersdorp and Karoo Supergroups. There are reported to be around 850 know kimberlite intrusions in southern Africa, of which only about 50 are diamond bearing, and very few contain diamonds in economic quantities.

Over the course of many millions of years, erosion removed the upper parts of the volcanic pipes and countless almost indestructible diamonds were shed into streams and rivers, where they are still being mined as alluvial diamonds; or they found their way to the Atlantic coastline, where they are recovered in abundance along the old beach deposits and out at sea. Before the discovery of the pipe at Kimberley, diamonds were known only from alluvial river gravels in India, Brazil and South Africa (the Vaal River). The mines of Kimberly showed for the first time that diamonds could also be found in a certain type of volcanic rock that formed steep, deep pipes. Here for the first time was the ‘primary’ source of diamonds. Diamonds formed in the mantle as volcanoes. This rock type, not to geology at the time, was identified and named ‘kimberlite’ by Carvill Lewis, (an American geologist).

The Kimberly region is underlain by thick continental crust of the ancient pre-Cambrian ‘Kaapvaal Craton’, which is the key to the genesis of diamonds. Diamonds form at a depth of more than 150 km in the upper mantle, but only within the relatively cooler diamond-stable zone that exists beneath thick cratons. The diamonds are brought to the surface as accidental inclusions in kimberlite magma of even deeper mantle origin, (pictured below).

Diamondiferous kimberlites are found within the oldest crust, (more than 2.5 billion years old). These are known as Archaean cratons. While exploring for diamonds, geologists look for diamond-related minerals, called indicators, because there are more of theses than diamonds, so they are easier to find. The more indicators there are in soil samples, the closer the source kimberlite is. The best indicators are pyrope garnet (orange, red and mauve), chromite and micro-ilmenite (black) and chrome diopside (bright green). Geophysical methods that measure physical properties like magnetics or gravity and detect kimberlites in suitably quiet backgrounds, are also used.

At surface the “Big Hole” is very roughly circular and about 500m across. Considerable slumping of softer sedimentary side walls into the hole has taken place, but the elliptical outline of the kimberlite pipe is clear in the deeper and harder Ventersdorp lava. In the cretaceous the explosive eruption of the Kimberley volcano probably formed a massive crater into which sediment was deposited and in which a variety of life existed. The upper part of the pipe was removed during many millions of years of ongoing denudation of South Africa’s interior plateau, which here amounted to the removal of an estimated 900m of overlying Karoo formations.

Pictured above, this is one of the world’s most famous kimberlite pipes, the smallest of the 5 large diamond-bearing pipes clustered around Kimberley. Roughly oval in shape, and typically carrot-shaped, it narrows as it goes down, from about 300m by 150m at the surface to around 170m by 70m at a depth of about 650m. This is the lower ‘diatreme’ portion of the pipe, and as it descends the narrowing pipe connects with an east-west-trending kimberlite dyke, the probable feeder fissure. This kimberlite is about 87 million years old; the diamonds that it contained are very much older.

Pictured above, the pipe was dug by hand to a depth of 240m using picks, shovels and explosives to create the Big Hole. Thereafter it was mined underground by a shaft system that reached 1,097m below the surface by the time the mine shut down in 1914. Some 22.5 million tons of rock were removed, from which over 14.5 million carats of diamonds were recovered. Both the grade and the quality of diamonds from this pipe are said to have been higher that those from the other Kimberly mines.

Picture above, when underground mining was introduced, the chambering technique was developed to access the kimberlite ore that had become impossible to extract through open-pit mining. Vertical or inclined shafts were dug some distance from the mine and then tunnels were hewn, intersecting the kimberlite pipe at depth.

Pictured above, at the end of these tunnels, vast chambers were hewn in the ore. Explosive, affixed to a long fuse, were rigged into the kimberlite ore and thus the diamond-bearing rock body was blasted. The ore crumbled into the chambers and then it was loaded onto cocopans and brought to the surface for sorting. [This chambering technique is the forerunner of today’s Blast Hole Open Stoping mining method]. Once ore had been extracted from one level, a new series of shafts and tunnels was constructed lower down and the extraction process was repeated. Each new set of chambers was therefore much deeper underground.

The secret to diamond’s superlative properties lies in the nature of its carbon bonding at high pressure and temperature. Carbon has four available electrons in its outer shell. In diamond, these electrons bond with four others form four adjacent carbon atoms, resulting in a tetrahedral unit of strong covalent bonds. The atoms are tightly packed together, giving diamond not only a high density (3.51), but also a repeating structural unit of eight atoms arranged in a cube. Because of the cubic symmetry of the atoms in the diamond lattice, diamond can grow into at least two different crystal shapes.

The common, well-known shape of perfect diamond growth is the octahedron (an eight-sided crystal). Cubes (six-sided) tend to grow when growth rates are fast. Twinned crystal are common, and most abundant are flat shapes known as mackles. Other shapes are just s common, but are not growth shapes; rather, these are shapes caused by the chemical dissolution of diamond. Most common shapes in diamonds from South Africa are rounded duo-decahedrons (12-sided). Typically caused by the chemical resorption of eight-sided octahedrons. Natural breakage is also common and this forms diamond fragments of no shape. The famous Cullinan Diamond was simply a large fragment of even bigger diamond.

Pictured above shows the geology in the upper part of the “Big Hole” excavation. The thin brown top layer is surface debris atop a weathered dolerite sill, followed by a thick layer of blueish Karoo (Ecca group shale) and thin Dwyka Group tillite that overlies the much harder solid vertical wall of Ventersdorp Supergroup lava. The water level has settled at around 175m below the land surface.

Pictured above, Cecil John Rhodes was a statesman and empire builder who had the greatest influence of any single person on the history of the African continent. The son of an English parson, he was sent to South Africa on account of his lung condition. Here he joined his brother, and they soon made their way to the diamond fields, where Rhodes gradually advance from being a speculative digger to forming De Beer Mining Company in 1880 and De Beers Consolidated Mines Ltd in 1887. He also acquired a large stake in the Transvaal gold mines and formed the Gold Fields of South Africa Company in 1887.

150 years ago, the site of the Big Hole was a featureless, flat-topped hill. When word spread that diamonds had been discovered, thousands of prospectors, armed with nothing more than picks, shovels and hope, descended on Kimberley and created the largest hand-dug excavation in the world. Kimberly is one of the most unique and authentic historical destinations in South Africa.

The Kimberley Mine Museum, located nearby, is a recreation of the town in its heyday. While visiting, we were able to go underground in a recreation of a mine shaft from the time period, watch a film about diamonds in Kimberley, visit an exhibition center, see a cut and uncut diamond display, use the new viewing platform to get a bird’s eye view of the Big Hole from above, and visit the Old Town to see Kimberley in its prime, (pictured below).