Since we are talking about geology, I do not want to give up the opportunity to bring up one of the coolest stories of geology ever, given the present day discussion of science and religion. You will be asking for a source for this story. Look it up in Wikipedia, where all knowledge resides, and you will not find it there.
There are things, it turns out, that The Great Knowing Web Site does not know. My source is a combination of primary and secondary documents, written histories, and a documentary that is not generally available.
Barney Barneto nee Barnet Isaacs was a key player in the historical development of the diamond industry of South Africa. Barneto, his acquired name, stands for "Barnet Too" which was his tag line when he worked as the secondary, added-on attraction in a magic act operated by his brother in South Africa. The act would be introduced ignoring him, and he's yell out "And Barnet Too." Barneto is one of two men, the other being Cecil John Rhodes. Yes, this is Rhodes as in Rhodesia, and this is the same man who led the British in Kimberley during the Siege. In fact, the private game reserve I mentioned earlier .... that was his.
Barneto and Rhodes would ultimately consolidate the myriad diamond claims in the Kimberley region. After Barneto and Rhodes had scarfed up most of the claims, Rhodes bought out Barneto's consolidated claims. The Megadiamondcorporation to which I earlier referred is the resulting company, and if you own a diamond, this corporation likely sold it to you. If you own an antique diamond more than a few decades old, there is a good chance it came from Kimberley.
It is said, and I think even Wikipedia may know this, that when Rhodes issued the multi-million dollar check to Barneto to acquire all of his claims, that instrument ... the check itself ... was the largest banking instrument ever issued to that date.
Anyway, Barneto did not simply acquire diamond claims. He acquired certain diamond claims. As I had mentioned much earlier, the average white South African believed that god had placed these diamonds here for the white man to attain wealth. The local black and other non-white South Africans had other stories which were typically much more poetic and typically less post-hoc, but no more scientifically correct.
I should mention that it was during this time that diamonds actually became the most valuable (more or less) gem. Indeed, there was another south African gem, called Tiger's Eye, which was considered at the time to be potentially more valuable and useful as a domestic use gem (like for wedding rings and stuff) than diamonds. Tiger's Eye comes from the Asbestos Hills Jasperite deposits I had mentioned earlier. It is said, and I have this on good but unsubstantiated authority, that a sample of Tiger's Eye had been sent back to Europe at around the time the diamonds were being discovered here. A return letter asked "How common is this Tiger's Eye gem? It is quite nice and potentially much more valuable than these plain, clear diamond rocks people are staring to ship here" or words to that effect. The answer sent back by a settler in the Asbestos Hills: "Oh, there's piles of it. It is quite common."
As a result, Tiger's Eye became nearly valueless and Diamond became the gem of choice, even though Tiger's Eye is considerably rarer than Diamond. Orders of magnitude rarer.
But that is a digression. I want to get back to Barneto, and then, eventually, on to the exciting end of this ghost story.
So, Barneto was busy buying up diamond claims in several localities that were under active mining. Most of the miners were content with a religious explanation for the diamonds being where they were, and that is important because it never occurred to anyone that there were at least three distinct types of material being dug to find the shiny little rocks. Outside of the volcanic plugs, and this was not being exploited much yet in those days but it is where the diamonds were first found, were gravelly deposits that are former river channels. Farmers who found these diamonds did not know that these were former river channels, because there is no reason for there to be former river channels on a landscape created as you saw it by God Himself. In the old volcanic plugs, there were two main types of deposit, a bluish earth and a yellow earth. The yellow earth was easier to dig, so all else being equal people tended to prefer claims ... and remember, these claims were tiny, like a few feet by a few feet in size ... that were primarily in yellow earth. Indeed, at the time Bernato was buying up claims, many sections of bluish earth were being dug around and were left standing in the ever-deepening holes that were being dug (The Big Hole was one of four that would eventually be mined in Kimberly, the last diamonds coming out in about 2005).
So somewhere along the line, Bernato came across a report written by a geologist. Now, you have to understand that geologists existed in those days, and had been busy working out geological questions for decades before any of this diamond mining was going on, but it seems to me that not much work was going on yet in South Africa. Certainly, the vast majority of human labor expended on the excavation of The Big Hole prior to about 1880 was a labor expended in a nearly science-free (but not engineering-free) environment.
Bernato's acquired report described a theory linking volcanoes, diamonds, and the deposits that were being dug right then in the Kimberley area. This scientific theory, which seemed to have a fair amount of consistency and with evidence to back it and everything, indicated that the place to look for the most diamonds was the blue earth, which eventually became known as "kimberlite." Kimberlite is the most pristine part of the earth's crust brought up from the deep by the volcanic magma. (I oversimplify slightly.)
Ironically, South Africa is now a region where it can be safely said that there is more geology per square mile than anywhere outside of Great Britain. The key point here is that Bernato ended up owning a huge share of the diamond mines because he used science. The other people ended up not owning that many diamonds, and for most people, actually ended up in one of the aforementioned graves that were dug daily at the edge of town, penniless and forgotten, because they thought The Almighty God had put the diamonds there for them.
Ghosts beget ghosts.
And peaking of ghosts, let's get back to the ghost story... but alas, we have once again run out of time. Next time, I promise, I'll tell you the story of how I single handedly captured the Ghost of the Infirmary.
The yellow rock is just weathered kimberlite. Kimberlite is not very hard to start with, and the yellow stuff will crumble in your hand. The volcanic plugs you mentioned earlier are probably from more typical volcanoes; the kimberlite pipes eroded down into depressions called "pans" where the first diamonds were found. I was diamond mining myself a couple of months ago. :) Didn't find any. :(
The yellow rock did not have diamonds in it (to the same degree) so I'm not sure that we are talking about the same thing here. Perhaps there is a reason you did not find any diamonds!
I may be full of it on other things (and feel free to say so!), but trust me on this thing. The yellow rock has just as many diamonds as the blue it came from. It is very crumbly, though, so you can basically mine it with a shovel and a sieve.
OK, you may be right, but that is in conflict with geological reports contemporary with the mining of the Big Hole in Kimberly, which was pretty much mined out before you were born. It is quite possible that we are talking about two different phenomena here.
Next time you are in South Africa stop in at the museum (at the Big Hole) in Kimberly and ask around!
The "yellow rocks" are, perhaps, weathered "blue rocks", and may have experienced physical displacement to some degree, and comprise placer type deposits, which would also suggest (again, perhaps) preferential sorting. The trick, then, when hunting diamonds in the yellow substrate, would be where you look within that substrate, as opposed to the blue substrate, wherein the diamonds are more isotropic, with regard to spatial distribution. The total diamond content, per volume, doesn't change, merely the distribution within the equivalent volume.
These would have been in situ deposits.
Interesting, I didn't know the diamond area in S. Africa was called Kimberly. The diamonds in Australia are also mined in a place called "The Kimberly".
Alan, Kimberley South Africa is the boom-town from the diamond days. The Australian region is named after Kimberley, South Africa.
Oh, ya. There are placer-type deposits where the diamonds have washed out of the pipe. The diamonds concentrate much like gold, as they are heavier than almost all of the other minerals. The pipes they originally come from are not really so rare, but they are hard to find as they are only a couple of hundred meters across and they just look like normal dirt when you walk across them. A little yellowish, maybe, but nothing sticking up like a volcano or any obvious signs of it from the surrounding geology. People mined the placer deposits for centuries without ever realizing where the diamonds came from.
CherryBombSim, that is partly true, but quite a few of the pipes stick up out of the surface. The big hole itself is one of them; It started out as a hill.
Placer deposits are the best localities to look, because as you say diamonds, being rocks and relatively heavy, can concentrate in gravel deposits. It can take a lot of work to find deposits that actually have diamonds in them. I've spent a fair amount of time poking around in the trenches dug by diamond prospectors (who use backhoes) and did a major geological/archaeological project on a place known as "Diamond Kojpe" which was obviously misnamed because there were none.
As the "Orange River" passes over and among the remains of volcanic plugs that have diamonds in them, the diamonds become part of the "lag" (gravel moving along the base of the river) but so does a lot of other stuff. There are plenty of other metamorphic rocks in the area that are hard and dense as well.
The river moving through a diamond deposit is much more likely to spread out diamonds than to concentrate them. But then, at a few key spots along a river you can get the reverse, where gravel more or less stays in place while softer bits are eroded away, lighter rocks drawn off, etc. and then there can be concentration. Those are the places to look for diamonds.
Locally people believe there may be concentrations of diamonds at the base of Augrabies Falls, but if anyone had gone in there to find out, they have not returned!