DIAMOND in SOUTH AFRICA
The richest of these deposits lie on both banks of that portion of the Vaal River flowing between the mission stations, Pniel and Barkly West (formerly Klipdrift), to the east, and Delport's Hope, at the junction of the Vaal with the Hart River, to the west, Barkly West being at the present day the center of the diamond-washing industry. In addition, diamonds have been found in small numbers further up the river at Hebron, and even as far as Bloemhof and Christiana in the Transvaal; also in the opposite direction, at the junction of the Vaal with the Orange River. A few diamonds have also been found in the Orange River, between its confluence with the Vaal and Hopetown, as well as in some of the tributaries of the Vaal, notably the Modder and the Vet. The yield in all these places was, however, so poor that the workings were soon all abandoned except the portion of the Vaal River mentioned above, and a stretch of its valley, parallel to the same portion of the river, and measuring fifty miles in a straight line, or seventy-two following the windings of the river. At the present day whole series of mines even in this region are practically deserted, the workers having left the river for the far richer dry diggings of Kimberley. The production of the river diggings up to 1871 was of some importance, but is now quite insignificant; in spite of the poorness of the yield, and the miserable conditions under which they have to work, a small number of a certain peculiar class of diamond-miners still cling tenaciously to their holdings in the hope no doubt of better days coming. Counting both black and white men, their number for many years probably did not exceed two or three hundred; they work singly or in twos or threes, not in large companies, and are most frequently to be seen in the neighborhood of New Gong-Gong, Waldeck's Plant, and Newkerke. The amalgamation of the "dry diggings" to form the De Beers Consolidated Mines, has had the effect of increasing the number of river diggers, it being estimated that there are now 1000 of them, exclusive of native workers. Companies have been formed with the object of working the deeper beds of river sand, but have met with little success. The river-diggings, on account of their poor yield, are known as "poor men's diggings."
The bed of the Vaal is strewn with blocks of basalt, often amygdaloidal in character, and with other rocks, which are probably of metamorphic origin. These blocks are usually of considerable size, and have been washed down from the sides of the valley and from the surrounding hills; between them lay a loose material consisting of gravel, sand, and mud, and it is in this that the diamonds are found.
The whole deposit, which varies in thickness up to 40 feet, rests on basalt, this rock being in situ and in here and there scooped out to form deep hollows, known as pot-holes or "giant-kettles", similar to those found in the beds of the diamond-bearing rivers of Brazil, which have been worn out by the continued whirling of pebbles in the eddies of the stream. The diamond-bearing debris accumulates in these depressions, which often yield a rich harvest to the finder.
The search for diamonds was at first confined to the bed of the river, but it was soon discovered that the sands and gravels of the river-terraces were as rich or richer than the riverbed, so that these also came to be worked. The terraces and their workings are usually only a few yards above the present high-water level, but one or two are 200 feet above this level. The workings in the river-terraces are easier to manage and more secure than those in the river-bed, since the latter are liable to be flooded, and thus considerably damaged; it has, therefore, been proposed in recent times that the stream should be diverted into another channel, but this scheme has never yet been carried out.
The diamonds found in this sandy clay are, as a rule, distinctly water-worn, though not of course to the extent of the other pebbles and sand grains which accompany them. These accompanying pebbles consist of various minerals, the different varieties of quartz, agate, jasper, silicified wood, etc. which have all traveled down from the upper courses of the river, being especially abundant. Pebbles of the rocks, which occur in situ in the neighborhood, are present in large numbers in these alluvial deposits. The minerals that are associated with the diamond in the dry diggings are less abundant, but small fragments of garnet, ilmenite ware met with. It is among these pebbles that the diamond is to be found; its distribution is, however, extremely irregular, a miner who hits on a favorable spot may make his fortune in a very short time, while his comrades toil on month after month unrewarded by the smallest success.
The method of work does not differ essentially from that followed in the diamond or in the gold-washings of the other countries. The sand and clay in which the diamonds and other pebbles are embedded, must first be excavated; this, when the diamantiferous material is overlain by blocks of basalt of considerable size, is no light task. This material is placed in a cradle, and the clay and fine particles washed away by rocking the cradle under a stream of running water; what remains after this process is put through a sieve, and the coarse residue, which contains the diamonds, is spread out on a sorting table and the diamonds picked out by hand. This final operation is easily performed, for the peculiar luster of diamonds enables a practiced sorter at once to distinguish them from other pebbles.
The yield was not very great, only on an average about 15,000 or 20,000 carats a year; in 1890, however, 28,122 3/8 carats, valued at £79,231 ($11,018,303), were obtained; a production of 30,000 carats (about 13 lbs. avoirdupois), is seldom reached, and never exceeded.
The quality of the yield in part compensates for its small quantity, stones from the river diggings being on the average far superior to those from the dry diggings. The average value of the former is in consequence much higher than that of the latter; for example, in the eighties a river-stone weighing one carat was worth 56s ($78), while a carat stone from Kimberley only fetched on an average 22s. 9d ($32).
A few specially large stones have been secured in the river diggings, such, for example, as the "Star of South Africa," a diamond of the purest water, weighing, in its rough condition, 83 1/2; carats; also the slightly yellow "Stewart", weighing 288 3/8 carats, which was found at Waldeck's Plant on the Vaal River.
The sands and gravels in which the diamonds are found in the river diggings are secondary deposits. It has been suggested that these sands and gravels have been derived from a deposit similar to that in which diamonds are now found in the dry diggings, and situated somewhere in the neighborhood of the source of the Vaal River. The denudation of such a deposit would supply the diamantiferous debris carried down by the river. That the diamonds have been transported some distance is shown by their distinctly water-worn character, and in all probability the original deposit was situated somewhere below Bloemhof in the Transvaal, since no diamonds have been found above this town. The fact that very few of the minerals associated with the diamond in the dry diggings occur in the Vaal River is easily explained when we consider that these minerals are not very hard and would be reduced to powder before they had been transported any great distance by the running water; whereas the harder minerals, found in the basin of the upper part of the Vaal, and now associated in the river deposit with diamonds, would resist the action of the water for a longer period and would be transported over greater distances.
Rafal Swiecki, geological engineer email contact
This document is in the public domain.