How Diamonds are Mined
Geology

Diamonds were formed many millions of years ago in the mantle of the earth, more than 150 kilometers below surface. Carbon present in the mantle was subjected to very high pressure and temperature and crystallized to form diamonds. Much later - geoligists estimate between 90 and 120 million years ago - melted rock or magma formed in the mantle, forced its way through the earth's crust and burst through to surface in volcanic eruptions. The magma carried diamonds with it - and when it eventually cooled in the pipes, the diamonds became entrapped in the rock that it formed. This became known as Kimberlite, named after the city of Kimberley.

There are 16 known Kimberlite pipes within a 10-kilometer radius of Kimberley, 6 of which are diamond bearing. Most of them have been mined with varying degrees of success in the past, but at present only two remain in production after more than a hundred years.

The larger pipes in the Kimberley cluster are oval-shaped and measure between three and 11 hectares at the surface. However, they do decrease in size with depth, so that they measure only between one and five hectares at a depth of 600 meters. Below this depth the pipes become highly irregular and fragmented in shape. There is an overall tendency for ore grade to decrease with depth, eventually making it too expensive to mine.


Mining Methods
Evolution of Mining Methods

Initially diamonds were mined along river banks. Later, when the primary deposits were exploited, the various mines were worked by opencast methods. Basically this means a huge hole was dug into the pipe by hand using primitive equipment . In the early days at Kimberley, the workings of numerous independent claim holders were crowded into the relatively small area of the pipe.

With the increasing depth of excavations, work became steadily more difficult and dangerous. Efficient, large-scale operations became possible on these pipes only when the small holdings were ultimately amalgamated into one company with the formation of De Beers Consolidated Mines Limited in 1888. Opencast work is normally abandoned when it is no longer possible to efficiently work the pipe from above. Underground work then commences.




Block caving
This is one of the mining methods used once underground work commences. Once a service shaft is sunk alongside the pipe, a series of concrete lined tunnels are punched through it, side by side, at 14-meter intervals across, beneath a solid block of Kimberlite about 180 meters high. From various draw points set at five-meter intervals, cones are cut into the still solid block of Kimberlite above to a height of nine meters. The block is then completely undercut immediately above the cones, causing the solid mass of Kimberlite to cave and break up under its own weight. Broken ground is drawn through the drawpoints into the drifts, where scraper winches remove it to the haulage. The ore is then hauled by electric train to ore passes leading to the underground crushing plant, which reduces the material to 150 millimeters in diameter. It is then conveyed to the skips for hoisting to the surface.

Alluvial Mining of diamonds:
This method encompasses very primitive manual operation to huge sophisticated earth moving operations. Ancient beaches are located and huge amounts of sand is removed to expose ancient sea-floors. Giant vacuum cleaner-like machines then scour the sea-floor and suck up the diamonds.