Diamonds are mined from three kinds of deposits: volcanic pipes of kimberlite or lamproite, riverbed and beach gravels, and sea-floor sediments. Pipe mines start as open pits and often move underground as they deepen. Alluvial and marine mines dig, dredge or suck up gravels that rivers and waves have already concentrated. All methods then crush and sort huge amounts of rock to recover a few carats of diamond per ton or less.
Where mined diamonds come from
Diamonds form roughly 150 to 200 kilometers down, in the mantle under old, stable parts of continents called cratons. Deep-source eruptions carry them to the surface in kimberlite and, less often, lamproite pipes. Over millions of years, erosion washes some of these diamonds into rivers and out to sea. The diamond geology page explains these processes.
Only a small share of kimberlite pipes contain enough diamonds to mine. Grades are often stated in carats per hundred tonnes. Many mines recover well under one carat per tonne of ore.
Open-pit mining
Most pipe mines begin as open pits. Crews drill and blast the kimberlite in benches, steps cut down the sides of the pit, and haul trucks carry the broken ore to a processing plant. The pit widens at the top to keep the walls stable, so large volumes of surrounding rock (waste) must also be removed.
The Kimberley mine in South Africa, the Big Hole, was dug largely by hand in the 1870s and later years before going underground. Modern open pits use giant trucks and shovels. Jwaneng and Orapa in Botswana are large open-pit operations, and Udachny in Russia was worked as a deep open pit before moving underground. The diamond mines of the world page lists major mines by region.
Underground mining
When a pit gets too deep for the waste removal to pay, mines often move underground to follow the pipe downward.
Block caving
Block caving is the main method for large underground diamond mines. Engineers dig tunnels beneath a block of ore and blast a layer, called the undercut, to remove support. The ore above then breaks and collapses under its own weight into funnels, where loaders collect it. Gravity does much of the breaking work, so block caving suits large, low-grade orebodies. The Cullinan mine in South Africa and the Argyle mine in Australia have used it.
Other underground methods
Smaller or narrower orebodies may use sublevel caving or sublevel stoping, where crews blast ore from levels in a planned sequence. Underground mining costs more per tonne than open pit, but it moves far less waste rock.
Alluvial mining
Alluvial deposits are diamonds that erosion has carried into riverbeds, terraces and floodplains. Rivers sort sediments by weight, so diamonds tend to collect in gravel beds and potholes in bedrock. Alluvial diamonds are often high quality, because weaker or flawed stones break up on the journey.
Alluvial mining ranges from large mechanized operations, which strip overburden and process gravels with heavy equipment, to artisanal digging with shovels, pans and sieves. Artisanal mining supports many people in countries such as Sierra Leone, the Democratic Republic of Congo and Angola. It is also hard to regulate, which has linked some alluvial production to smuggling and conflict (see conflict diamonds).
Historic sources were alluvial: India's Golconda region, Brazil's Minas Gerais and Borneo all produced from river gravels. See the Golconda mines.
Marine and coastal mining
The Orange River carried diamonds from inland southern Africa to the Atlantic coast for millions of years. Waves and currents spread them along beaches and onto the sea floor off Namibia and South Africa.
Coastal mining in Namibia has involved building sea walls to push back the ocean, then stripping sand to reach diamond-bearing gravels on bedrock. Offshore, specially built ships use seabed crawlers or large drills to bring sediment up to processing plants on board. Diamonds recovered this way have a very high share of gem-quality stones, because the long journey destroyed weaker crystals. The diamond mines of Namibia page covers the Sperrgebiet and marine operations.
Processing the ore
Mined ore goes through several stages to separate the diamonds.
- Crushing. Crushers break kimberlite into smaller pieces, taking care not to break large diamonds.
- Scrubbing and screening. Water and rotating drums clean the ore, and screens sort it by size. Fine material is discarded.
- Dense media separation (DMS). Ore is mixed with a slurry of ferrosilicon and water that has a set density. Diamonds and other heavy minerals sink, while lighter rock floats away.
- X-ray recovery. Diamonds fluoresce under X-rays. Machines detect the glow and fire a jet of air to knock each diamond off the conveyor into a collection bin.
- Final sorting. Workers or machines in secure rooms pick out the diamonds, clean them and sort them by size and quality.
Older plants used grease tables, since diamonds stick to grease while wet rock slides off. Some plants still use grease belts for diamonds that do not fluoresce well. Many mines now also use laser and imaging systems to recover large stones before crushing could break them.
Environmental and community issues
Large mines move huge amounts of rock and water. Open pits leave large holes, and processing produces tailings and fine waste that must be stored safely. Mines can affect groundwater, rivers, wildlife and local land use. Modern mining laws in many countries require environmental assessments, water treatment, monitoring and plans to rehabilitate land after closure.
Arctic mines in Canada and Russia face their own challenges, including permafrost, seasonal ice roads and sensitive tundra. Marine mining disturbs the seabed in the areas it works.
Community issues include land rights, employment, revenue sharing and the effects on towns when mines close. Producer governments have pushed for local ownership, processing and cutting. Artisanal mining raises concerns about child labor, unsafe pits and low pay for diggers. The Kimberley Process addresses conflict financing but not most labor or environmental issues, which buyers raise through other channels. After recovery, rough enters the trade described on the diamond pipeline page.
Frequently asked questions
How are diamonds separated from rock?
Mines crush the ore, separate heavy minerals in dense media, then use X-ray machines that detect the glow of diamonds and blow them off the conveyor.
What is block caving?
It is an underground method where miners undercut a block of ore so it collapses under its own weight into funnels, where it is collected.
Why are marine diamonds high quality?
Diamonds carried by rivers to the sea and tumbled by waves lose weaker and flawed stones along the way. The survivors are mostly strong, gem-quality crystals.
How much rock is mined for one diamond?
It varies widely by mine. Many kimberlite mines recover less than one carat per tonne of ore, so tonnes of rock are processed for each carat recovered.