Diamonds are a mineral composed of carbon atoms arranged in a specific crystal lattice, making them the hardest known natural substance on Earth. Formed under immense heat and pressure deep within the planet, diamonds have been highly valued for their beauty and durability for centuries. Their journey from Earth’s depths to the surface involves complex geological processes and sophisticated extraction techniques.
Where Diamonds Are Formed and Found
Diamonds originate deep within the Earth’s mantle, typically at depths ranging from 150 to 200 kilometers (93 to 124 miles) below the surface. Here, temperatures can reach between 900 and 1,300 degrees Celsius (1,652 to 2,372 degrees Fahrenheit), and pressures are approximately 45 to 60 kilobars, conditions necessary for carbon atoms to crystallize into diamonds. These extreme conditions are present in ancient, stable parts of continental crusts known as cratons.
The primary way diamonds are brought closer to the surface is through explosive volcanic eruptions that form structures called kimberlite and lamproite pipes. These carrot-shaped conduits act as natural elevators, transporting diamond-bearing rock rapidly from the mantle to the Earth’s crust. Once these pipes are discovered, they represent a direct source of diamonds, often leading to large-scale mining operations.
Over geological timescales, diamonds from these primary kimberlite and lamproite pipes can be eroded from their original locations and transported by rivers and glaciers. These secondary deposits, known as alluvial deposits, are found in riverbeds, floodplains, and ancient river channels, sometimes hundreds of miles from their original source. Additionally, some diamonds have been found in marine deposits, having been carried by rivers into oceans and settled on the seabed. The specific geological setting where diamonds are found significantly influences the methods used for their retrieval.
Extracting Diamonds from the Earth
The extraction of diamonds employs various methods, each tailored to the specific geological deposit. Open-pit mining is often used for kimberlite and lamproite pipes close to the surface. This method involves removing successive layers of rock and overburden to create a large, terraced pit that spirals downwards. Massive excavators and haul trucks move vast quantities of earth and ore from the expanding pit, which can extend to depths of several hundred meters.
For deeper kimberlite and lamproite pipes, underground mining techniques become necessary, as open-pit mining becomes economically and structurally unfeasible. One common underground method is block caving, where a network of tunnels and shafts is constructed beneath the diamond-bearing ore body. Sections of the ore are then undercut, causing the rock above to progressively fracture and collapse under its own weight, allowing the broken ore to be collected through extraction tunnels. This method requires precise engineering to ensure controlled and safe rock movement.
Alluvial mining focuses on recovering diamonds from secondary deposits found in riverbeds or ancient river channels. Techniques vary from manual methods, where workers sift through gravel using screens and water, to more mechanized approaches. Larger operations might use dredges that scoop up diamond-bearing gravel from river bottoms or utilize earthmoving equipment to excavate ancient river terraces. The excavated material is then processed to separate the diamonds.
Marine mining represents a specialized approach for recovering diamonds from the seabed, primarily off the coast of Namibia. This involves highly specialized vessels equipped with advanced technology. Remote-controlled crawlers or large drills are deployed to the ocean floor to suction or excavate diamond-bearing gravel from depths of up to 150 meters (490 feet). These operations require sophisticated navigation and surveying systems to precisely locate and extract marine sediments.
Initial Processing of Raw Diamonds
After diamond-bearing material is extracted, it undergoes initial processing to separate diamonds from host rock. The first stage involves crushing and screening the ore. Large chunks are fed into crushers that break them into smaller, manageable pieces, calibrated to avoid damaging diamonds. This crushed material then passes through screens to sort it by size, preparing it for subsequent separation.
Following crushing, the material proceeds to dense media separation (DMS), a technique exploiting the density difference between diamonds and most other minerals. The crushed ore is mixed into a liquid slurry, often containing ferrosilicon powder, creating a medium with a specific density. Denser diamonds sink, while lighter waste materials float, allowing initial separation of the diamond-rich concentrate. This process efficiently reduces the volume of material needing further processing.
Further refining involves X-ray separation. This technology utilizes diamonds’ property to fluoresce, or emit light, when exposed to X-rays. As the material passes through an X-ray sorter, a sensor detects emitted light from diamonds, triggering a blast of air or water to divert the diamond into a collection bin. This automated process is highly effective in isolating diamonds from other non-fluorescent minerals.
The final stages of initial processing involve hand sorting and thorough cleaning. The concentrated material, now enriched with diamonds, is visually inspected by trained sorters. They meticulously pick out the rough diamonds. The collected rough diamonds are then cleaned to remove any remaining dirt or residue, making them ready for evaluation, grading, and eventual cutting and polishing.