Of all the diamonds mined in the world each year, less than half are gem quality; the rest fall into two other main categories known as near-gem quality and industrial quality diamonds.
Gem quality diamonds display a high standard of excellence in quality and are used in diamond rings and other diamond jewelry. The clarity of these loose diamonds ranges from flawless through to visible inclusions.
Near-gem quality diamonds represent those stones of a quality between gem and industrial that in fact can be used as either depending on the individual stone. These stones have clarity grades ranging from visible inclusions through to industrial.
Industrial quality diamonds are low quality or badly included stones and are suitable only for industrial use; for example, they are used in dentist’s drills and earthmoving equipment.
Diamond Mining
Diamonds are recovered by way of pipe or alluvial mining.
Pipe Mining
Pipe mining refers to the extraction of diamonds from volcanic pipes. Typically, a very large area has to be covered. An average of 250 tons of ore must be mined in order to produce a one-carat gem quality polished diamond.
In most countries, a diamond pipe mine is composed of kimberlite, or blue ground. Initially kimberlite is dug from the surface of the pipes in rough opencast mining. Once the surface deposits have been exhausted, shafts are sunk into the ground at the edge of the pipes, and tunnels are driven into the deeper parts of the pipes.
In open cut mining, the ore is dislodged by blasting and then loaded by excavators into 120-ton dump trucks. The ore is then transported to the processing plant where the diamonds are extracted. The processing techniques are purely physical and involve crushing, scrubbing, screening and gravity separation of the diamond-bearing ore. Final diamond recovery is achieved by the use of x-ray sorting machines. The machines can detect and remove diamond material because the diamonds fluoresce under x-ray.
Alluvial Mining
This process involves the extraction of diamonds from riverbeds or ocean beaches. Millions of years ago, at the time the diamond pipes were formed, some diamonds were weathered out of the pipes and carried great distances along rivers and even into oceans.
In order to extract these diamonds from beaches, a wall is built to hold back the surf. Up to 25 meters of sand is bulldozed aside to reach the diamond-bearing level. Once reached, the diamond-bearing earth is removed and transported to screening plants.
Screening Process
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
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.
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.
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 (blue) into a collector bin.