DIAMOND in SOUTH AFRICA
Dry Diggings (suite)
An exact record of the yield of each of the Kimberley mines has also been kept since September 1, 1882. The unknown, but probably very considerable, number of diamonds stolen by the workers from their legitimate owners cannot of course be included in these records; the value of the diamonds misappropriated every year is variously estimated at from £500,000 to £1,000,000. During the three years between September 1, 1882, and September 1, 1885, the four Kimberley mines, from which, as has already been mentioned, over ninety per cent of the total South African output is derived, have yielded, according to official returns, the following amounts in carats:
This gives the yearly average for the four mines together at 2,372,809 3/8 carats, valued at £2,628,289 3s. 7d. ($365,493,483)
In the year 1886, the production of these four mines, and of a few others of less importance, as well as of the river diggings, amounted to:
The figures in the three tables given above are in all probability too low, since there are many sources the supply from which it is difficult or impossible to estimate. Thus the total production of the river diggings, and of the Jagersfontein and Koffyfontein mines in the Orange River Colony, is not exactly known, the numbers in the table referring not to the total production, but to the diamonds sent from these mines and diggings to the central market at Kimberley. The first of the above three tables gives the total yearly production for the whole of South Africa, from the discovery of diamonds in 1867 up to the year 1892. Details connected with the production in certain years of individual mines are collected together in the second and third tables.
Formerly, when the claims were in the possession of single individuals or small companies, the aim of the owners was to obtain and sell as many stones as possible. Now, however, the output is controlled by the "De Beers Consolidated Mines," and only a certain number of stones are placed on the market, in order to keep up the price. Experience has shown that the annual demand for diamonds, both for jewellery and for technical purposes amounts in value to about £4,000,000. Taking the average value of rough stones at 21s. per carat, this corresponds to a weight of rather over 3,800,000 carats, or about 15 1/3 cwts. ; the yearly production of Cape diamonds is rather over 3,000,000 carats, and the difference between these weights represents the amount supplied by Brazil, India, Australia, and Borneo.
Although the total weight of stones collected is so enormous, yet the relative amount of diamond in the "blue ground" is extremely small. The proportion which this constituent bears to the whole mass varies both in different mines and in different parts of the same mine. Thus in some mines the "blue ground" excavated from successively lower depths has shown a marked increase in richness. It would be naturally of great importance to those interested, were it possible to foretell whether such an improvement in yield would be maintained or not; this, however, is not possible, and it is equally impossible to give any explanation of the variation.
The Kimberley mine, since its discovery in July 1871, up to the present day, has been the richest of all the deposits. Many of the original miners made their fortunes in less than a month, and one case is quoted in which diamonds to the value of over £10,000 were found in the short space of a fortnight. A cubic yard of "blue ground" from the Kimberley mine contains more diamonds than a mass of equal size from any other district; this being so, mining operations were here prosecuted with great vigour, all the other mines being at one time forsaken. Owing to the want of reliable statistical reports dealing with early times, it is not possible to determine with certainty whether the deposit increases in richness as greater depths are reached, or not. Probably it does not, and if this were so, the Kimberley mine differs in this respect from the others.
Boutan has collected statistics dealing with the content of diamonds in the "blue ground" excavated from the Kimberley mine by certain companies. From the year 1881 to 1884, this varied between 3. 04 and 7. 17 carats per cubic metre (about 13 cubic yards), an amount corresponding to two- to five-millionths of a per cent. This yield, however, applies only to the richest parts of the "blue ground," the mean yield for that part of the deposit, which is worked, taking rich and poor together, is about 4. 55 carats per cubic metre (3. 5 carats per cubic yard). If, however, the material of the western side of the mine, which on account of its poverty is not worked, is also included, the mean yield for the whole deposit will be 4. 20 carats per cubic metre of "blue ground," or on the average three-millionths of a per cent.
The dimensions of the portion of the deposit, which is worked being known, it can be calculated that the excavation of "blue ground" to a depth of one metre would result in the production of 88,000 carats of diamonds, which at £1 ($139) per carat would be of the aggregate value of £88,000. From the same part of the deposit, a cubic metre of worked material would contain on an average £4 11s. worth of diamonds.
The De Beer's mine is 115 times the size of the Kimberley mine; the central point of the former is situated 1,771 yards to the east of the central point of the latter. The deposit ranks next to that of the Kimberley mine in richness, although it was at first very poor, only yielding a carat to the cubic metre of rock. The yield, however, increased rapidly as greater depths were reached; at a depth of 300 to 400 feet it had increased tenfold, so that 3 1/2 carats of stones were then obtained from a cubic metre of rock. From the year 1882 to 1884 the yield obtained by some companies varied between 1. 28 and 3. 52 carats; the mean yield for the whole mine was estimated at 3~15 carats per cubic metre of rock, worth, at the higher price commanded by stones from this mine, £3 9s.
Rafal Swiecki, geological engineer email contact
This document is in the public domain.