The diamondiferous area is an extensive one. It is a strip of sandy country near the coast, from 2 to 12 miles wide, extending intermittently from Conception Bay (100 miles south of Swakopmund) to Angra Juntas, some 60 miles north of the Orange River, a total distance of about 250 miles. The strip is broken by a chain of hills and rocky ridges running mainly from north to south. In the wide valleys and depressions thus formed, ranging from 2 to 3 feet above sea-level to over 500 feet, the diamondiferous gravel is found. The deposits are by no means uniform. Large stretches of ground may not contain a single stone, while a rich “pocket” may hold scores of the glittering gems. The patch, too, that is so rich in diamonds may have a surface view precisely similar to that of the barren areas around. Such freaks of deposit seemed to some of the early prospectors to be the work of whimsical genii.
The precious stones lie among tiny fragments of banded agate, red garnet, red jasper, chalcedony, milky quartz, and sand.
The deposit varies in depth from 6 inches to 15 feet. Over the mixture the furious trade winds from the south rage for eight or nine months in the year. A process of natural concentration proceeds apace. The light particles are caught up and whirled away to the sand dunes, until in many places nothing is left but the heavy diamonds and a thin layer of coarse particles. Naturally, the little depressions here and there, especially those on the windward side of obstacles, have a good concentration of rich detritus. The gems are never found in any quantity in the valleys that run from east to west, but in those that lie in the line of the prevailing wind.
The diamonds found in this sand are peculiar to the country. They are wholly unlike any other known African stones. When in 1901 some natives professed to have found certain small stones in the alluvial diggings on the Vaal River, the experts knew at once they were not river stones. The boys had stolen them from German South-West Africa. All shades of colour are found among them, but the stones of a clear white appearance, with a barely perceptible yellowish tinge, predominate. Pale pinks and lemon yellows are fairly common. Impure shades are remarkably few, and fully 85 per cent. of the gems are fit for cutting. They are said to resemble the stones derived from Brazil. In size they are small; it takes six or eight to make a carat as a rule, but a few large stones have been found. One weighed 34 carats and another 17 carats. These large stones, however, are very exceptional.
How did the diamonds get there? That these lustrous gems should sprinkle the sand so thickly in this dreary region may well give cause for wonder. Geologists differ as to their probable source of origin. Dr. Wagner, in his exhaustive work on “The Diamond Mines of Southern Africa,” summarises the main theories as follows:
(1) The diamonds were released by weathering from the crystalline rocks of the basement system.
(2) The diamonds were derived from the denudation of the primary deposits of British South Africa, carried down to sea by the Orange River and distributed along the coast by the agency of the Benguella current.
(3) A modification of the second hypothesis, according to which the diamonds were carried down to the sea from sources believed to exist within the interior of German South-West Africa.
(4) The parent rock of the diamonds lies submerged off the present coast.
Dr. Wagner dismisses the first three, and advances arguments in favour of the fourth. He concludes that they have been derived “from a primary deposit, or from primary deposits, which now lie buried beneath the sea somewhere off Pomona,” as there is a steady—if not quite persistent—increase in the average size of the stones as one proceeds from north to south, until the Pomona area is reached, where the average weight is greater than anywhere else. On this supposition the lighter stones have been swept northward by a strong ocean current when the coast was still submerged. To this we may add the statement of Dr. Marloth that among “the prospectors who know the country south of Prince of Wales Bay, the belief is quite common that Pomona diamonds came from some volcanic fissures that occurred there.” Kimberlite “pipes” and dykes occur in the Keetmanshoop, Gibeon and Bethany districts, but they contain no diamonds.