Commonly a diamond mine will start off as an open pit dug out in a benched (or staircase) shape that can accommodate a sloping roadway for trucks to drive down to the bottom of the excavation. This design also prevents cave-ins.
Underground mining consists of a large vertical shaft running next to a kimberlite pipe to be used for access to many horizontal tunnels leading into the kimberlite. Diamond-bearing rock is blasted loose at the tunnel faces, hauled to the surface, and processed. Most of the mines vary in types of production. Some produce mostly industrial, others may produce higher qualities. Sometimes tons of rock, gravel or sand have to be processed to retrieve one carat of rough diamond.
The block-cave method is more efficient and more commonly used today. This is simply digging a cave underneath the kimberlite rock until it falls in. The crumbled gravel is more easily processed after it is funneled out into the collection areas.
Bench mining (or benching) is a process of blasting benches or slots in the sides of the open pit area where the kimberlite pipes lay. The gravel is directed down through funnels to the below-ground processing area. It is then crushed into pieces of six inches or less before it it hauled to the surface for further processing.
Mining is difficult, dirty, and dangerous work, requiring the movement of thousands of tons of rock everyday to make it all worthwhile. Some mines may only produce a single one-carat finished diamond out of 250 tons of rock. Eventually the yield of a pipe no longer supports the costs of mining and the operation must be closed down. The Kimberly Mine was 3,601 feet deep when it was closed.
A typical recovery processing method today involves first crushing the gravel into pieces measuring about 11U4 inches across. It is then forced across a series of screens called grizzlies, which separate the larger and smaller chunks. The small pieces of kimberlite are placed in a mechanical rocker box, where rotating paddles circulate water to separate the lighter and heavier minerals. The lighter materials eventually float off and the heavier diamond-bearing gravel collects at the bottom of the box.
One of diamond’s characteristics is its high affinity for oil or grease. Since before the turn of the century, grease has been used for sorting out diamond in such inventions as the grease table. The separated gravel is washed over a table covered with a thick layer of grease, leaving diamond rough behind, adhering to the table. The grease belt, a more modern version of the earlier grease table, uses a greased conveyer belt for continuous processing. Again, the diamond-bearing gravel is washed across the belt and the remaining rough is collected at the far end of the conveyor.
A major advance in the recovery systems came in 1958, when Soviet scientists invented the X-ray separation system. Relying on the fact that diamonds fluoresce under X-rays, the concentrated gravel passes through X-ray beams. When fluorescence occurs, an intense air stream is triggered that forces the diamond rough into a separate collection area.