The discoveries of the first two primary occurrences of diamonds that is, in pipes were made within 500 meters of each other in late 1869, a considerable distance (about 25 km) away from the wet diggings mentioned above or from any other obvious watercourse. Because of the lack of water in the vicinity of these occurrences, they were referred to as "dry diggings" to contrast them from the "wet diggings" however, no geological distinction between the two types of occurrence (i.e., secondary diamonds deposit vs. primary diamond deposit) was initially recognized.
As diamond mining progressed, it was soon realized that the geological setting of the dry diggings was different. Rather than being in sands and gravels typical of rivers, these diamonds were recovered from surficial material that was physically distinct (i.e., it was soft and had a high clay content) from that in the wet diggings. The material in the dry diggings became known as "yellow ground" (near-surface, oxidized and weathered kimberlite with yellow diamonds).
As mining continued deeper, the yellow ground was found to overlie "blue ground" (unweathered kimberlite, see below) which, because it was harder than the yellow ground (as well as the loose, unconsolidated sediments of the wet diggings), was much more difficult to mine. These new discoveries eventually became the Bultfontein and Dutoitspan pipe mines (that arc still in production, in 1995, 125 years later!).
They are two of the approximately twenty diamond pipes in the Kimberley area of South Africa, but only a few others (mainly the De Beers, Kimberley and Wesselton mines) have ever been economic to mine. The unusual nature of these deposits was further confirmed by the fact that as mining progressed the diggings took on the shape of steep sided cylindrical columns, which today we clearly recognize as the characteristics of a pipe.
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