The geological starting point: kimberlite pipes
Most of the world's diamonds come from kimberlite pipes, ancient volcanic intrusions where magma from the earth's mantle was forced rapidly through the lithosphere, carrying diamonds (formed at depths of 150–200km, temperatures of 900–1,300°C, and pressures of 45–60 kilobars) to or near the surface. The magma cooled and solidified into a distinctive rock called kimberlite, named after Kimberley, South Africa, where the first kimberlite pipes were identified as diamond sources in 1871 (historical documentation of kimberlite discovery; geological research on kimberlite pipe formation).
Kimberlite pipes are generally carrot-shaped in cross-section: wide at the surface (from a few hundred metres to over a kilometre across) and narrowing with depth. The diamond-bearing ore is distributed throughout the pipe, typically at concentrations of 0.5 to 3 carats per tonne for commercial operations. Most kimberlite pipes contain no diamonds or sub-economic concentrations; only a very small fraction of the tens of thousands of kimberlite pipes discovered globally contain diamonds at economically mineable grades (De Beers Group mining research documentation; geological survey data).
Open-pit mining
Open-pit (or open-cast) mining is used when the diamond-bearing ore is accessible from the surface without excessive stripping of overlying waste rock. The Jwaneng mine in Botswana is one of the world's largest open-pit diamond mines, an enormous excavation operated by Debswana, the 50/50 joint venture between De Beers Group and the Government of Botswana. Open-pit mining involves drilling and blasting the ore in benches (horizontal levels), loading the blasted material with large shovels or front-end loaders into trucks, hauling it to the processing plant, and crushing and screening it there (Debswana Annual Reports, debswana.com; De Beers Group mining documentation).
Underground mining
As open pits deepen, the economics shift. The ratio of waste rock to ore (the strip ratio) increases as pits deepen, eventually making open-pit operation uneconomical. At this point, mines transition to underground operations. The Cullinan Mine (formerly Premier Mine) near Pretoria, South Africa, now operated by Petra Diamonds, transitioned to underground mining and uses block caving: large sections of ore-bearing kimberlite are undermined, causing them to fragment and collapse into collection points for hoisting to the surface (Petra Diamonds Annual Reports, petradiamonds.com; mining methodology documentation).
Diamond recovery: screening, DMS, and X-ray sorting
Once ore reaches the processing plant, diamonds are recovered through a sequence of separation processes based on their unique physical properties, size, density, and X-ray fluorescence.
Screening: Ore is screened to remove oversized material and classify by size. Diamonds above a certain minimum size (typically 1mm) are captured; fines below this threshold are discarded.
Dense Media Separation (DMS): Crushed, screened ore is fed into a tank of heavy medium, ferrosilicon suspended in water, at a specific density. Diamonds (density approximately 3.52 g/cm³) sink; lighter minerals float and are separated. This step considerably concentrates the diamond-bearing material before more precise separation (De Beers mining methodology documentation; industry processing descriptions).
X-ray luminescence sorting: Diamond recovery at modern mines relies primarily on X-ray luminescence, diamonds glow (fluoresce) brightly under X-ray radiation, while most other minerals do not. Concentrated material from DMS passes on a conveyor under X-ray beams; sensors detect fluorescence; air jets eject the fluorescing (diamond) particles into a separate stream. This is the critical recovery step at most modern operations (De Beers Group Diamond Recovery Technology documentation; De Beers Group Annual Reports).
Primary sources
Debswana Annual Reports. debswana.com. [Jwaneng mine ore processing volumes (~12M tonnes/year); carats recovered (~12–14M ct/year); open-pit operations; Botswana government 50/50 structure.]
De Beers Group mining documentation and Annual Reports. debeersgroup.com. [Kimberlite pipe geology; open-pit and underground methods; DMS and X-ray luminescence recovery technology; diamond concentration grades (0.5–3ct/tonne commercial).]
Petra Diamonds Annual Reports. petradiamonds.com. [Cullinan Mine underground block caving methodology; Premier Mine historical context.]