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November 23, 2004
Diamonds from the Big Hole.
The Big Hole in Kimberley is unusual in that it was (and is!) the throat or magma vent of a long extinct volcano. The distinctive "blue" rock from this is called "Kimberlite" and it is now recognised as the prime diamond bearing rock everywhere. Until this "throat" was discovered and recognised as the source of the alluvial diamonds that littered the area and the river beds around here, almost all diamonds were found by chance as alluvial deposits. As far as I am aware the last volcanic activity in this area was some 200 million years ago and since then, of course, things have been weathering down quietly, mountains becoming hills, and even the hills becoming little more than weathered outcrops of rock.
The first large diamond found in this area actually came from Hopetown around a hundred or so miles to the South and East of Kimberley. It was a large stone and was eventually bought and cut into a number of smaller "brilliants". That started the rush of prospectors into the area and it was not long after that the diamonds were traced to a source where there is now the "Big Hole".
As Kimberley lies on a confluence of rivers (in the triangle formed between the Vaal and Orange Rivers) it eventually occured to someone that many diamonds must have been washed down these water courses. Bonanza! de Beers soon figured out what was likely to have happened and got control of the mineral and prospecting rights all the way to the coast. And they hit paydirt in the big league. The river bed has almost no diamonds now, but the coast at the mouth of the river and the seabed off that coast has around 150 million years worth!
The "Forbidden Territory" around Oranjemund, is now mined using huge excavators which shift the accumulated sand and expose the cracked and fissured bedrock. Then vast vacuum cleaners are used to suck up the sand and grit from these fissures and the gravel is sorted yielding quantities of diamonds. Most are commercial grades, but a few, a very few - but enough to make it pay - are gem quality. At sea off the coast are the diamond dredges. These huge barges dredge the sand and gravel from the seafloor - and again the diamonds are sorted and retained while the rest is dumped.
Speaking to an engineer who kept the dredges running, I learned that they watched the gravel for signs of a greenish rock which is the oldest form of rock - one formed in the earliest days of the crust forming as this planet changed from molten lump of steel and magma into a planet of rock. When this gravel showed, there would be diamonds among it.
It took the Kimberley "pipe" almost 200 million years to give up the diamonds that it probably took 400 million years to form. This part of South Africa was a seabed around 300 million years ago, and under an ice sheet around 450 million years ago. Further South the mountains still show the traces of glacial activity and all around Kimberley the age of the rock, the slow destruction of the ancient landscape by the forces of nature, are in evidence.
Next time you see a diamond, consider the length of time it took to form, the forces involved - and the effort it has taken to retrieve it. Perhaps it puts our own achievements into something of a better perspective. It certainly makes us aware of just how short a time we have been around - or had the capacity to retrieve these precious and special stones and polish them into decorative gems for our delight. The mere blink of an eye in terms of the diamond itself.
Posted by The Gray Monk at November 23, 2004 03:03 PM