DIAMOND in SOUTH AFRICA
Dry Diggings (suite)
Exchange rate between the old and present value is: £1 = US$140.
The richness of this mine varies not only with the depth, but also at different places on the same level. The best part of the deposit is not surpassed in richness by any part of the Kimberley mine, while other parts are so poor that they are not worked at all. The central portion is very rich, and extensions of this stretch out especially towards the north and south, forming a great contrast to the western third of the mine, which is extremely poor. A beautiful yellow octahedron, weighing 302 carats, was found in the eastern side of the deposit, on March 27, 1884.
The increased richness of the lower lying parts of the deposit led to an attempt being made, soon after the opening of the mine, to excavate the deeper portions by means of underground workings, leaving the poorer portions standing above. The attempt was very successful as far as yield was concerned, but owing to the imperfect methods adopted, was attended by so many accidents that it had to be abandoned. Since 1885, however, when, as we have already seen, the construction of underground workings began to be more skilfully engineered, the excavation of the deeper lying material has been resumed.
The Bultfontein mine is situated 4,840 yards to the southeast of the Kimberley mine. A cubic metre of the surface material yielded only a small fraction of a carat, but here also a rapid and regular improvement in the deposit as lower depths were reached was manifested, the yield at a depth of 200 feet having increased threefold. The increase in the richness of the deposit took place in this mine with almost mathematical regularity, and was attended both by an improvement in the quality of the stones, and by a diminution in the number of fractured crystals. In 1887 a depth of 460 feet had been reached, a depth, which has never been exceeded in open workings. In the period 1881 to 1884 the yield varied between 0.56 and 1.27 carats per cubic metre of "blue ground," the mean yield being about 1.05 carats, so that a cubic metre of rock contained diamonds to the value of about 23s.
Du Toit's Pan mine is 1,320 yards distant from Bultfontein mine, and 3,542 from De Beer's mine. In 1874, the mine was almost deserted, the yield being so small, and it has only been systematically worked since 1880. Here again the surface rock was poor, and yielded at the best of times only 0.25 carat per cubic metre of material; here also the deposit improved at greater depths, but not so rapidly as in the De Beer's mine. At a depth of 175 feet, at which the yield had doubled itself, a peculiarity not hitherto noticed in any other mine was observed: the richness of the deposit was found to be absolutely identical at all points on the same level, so that no variation in the yield was experienced. From this point down-wards the yield rapidly increased, approaching that of the Kimberley and De Beer's mines. From 1881 to 1885 the yield varied between 0.31 and 1.11 carats, the mean yield being 0.77 carat per cubic metre of "blue ground", having the average value of 22s. In this mine, as in the Bultfontein mine, the increase in richness of the deeper lying deposit was accompanied by an improvement in the quality of the stones, and by a diminution in the number of broken crystals. The open workings were here excavated to a depth of over 400 feet, and underground workings have been scarcely as yet commenced.
The Jagersfontein mine, near Fauresmith, in Orange River Colony, contains only 0. 10 to 0. 35 carat of diamond per cubic metre of "blue ground". The poorness of the yield is, however, in some measure compensated for by the singular beauty and size of the stones. For the year ending March 31, 1891, the average value per carat of stones from this mine was 37s. , stones from the Kimberley mines being worth only 25s. 6d. per carat. The largest diamond known was found in this mine in 1893; it weighed 971 3/4. A very fine stone of 655 carats was found here also at the end of 1895. The mine was opened in 1880, was abandoned for a time about 1885, but was subsequently re-opened.
The Koffyfontein mine, also in the Orange River Colony, gives a smaller yield still, amounting to only about two-thirds that of the Jagersfontein. The stones, however, are of good quality and are worth about 30s. per carat; from December 1887 to April 1891, 9912 carats of diamonds, valued at £14,640, were mined here.
The relative importance of the different mines is also shown to a certain extent by a comparison of the number of workers employed in each. Thus in the year 1890 the numbers were as follows:
The number of persons employed in the mines at Kimberley in the year 1892 is given in the following table. This includes two mines not before specially mentioned, namely Otto's Kopje and St. Augustine's, the former being situated a couple of miles to the west of Kimberley, and the latter in the town itself.
We turn now to the consideration of the characters and the quality of Cape diamonds, that is to say, the form and condition of crystallization, colour, size, etc. peculiar to stones from this region.
These diamonds usually occur as distinct crystals, symmetrically developed in all directions and with perfectly sharp edges and corners; but fragments of larger crystals, hounded by cleavage surfaces, and which are therefore cleavage fragments, also occur with considerable frequency. These cleavage fragments are sometimes of fair size, the original crystals, of which these fragments are part, probably varied in size from 3 to 500 carats; large cleavage fragments are known as "cleavages," while fragments weighing less than a carat are referred to as "splints". It is a remarkable fact that these cleavage fragments are nearly always white, that is colourless, or at least very faintly coloured; fragments of a dark colour, or of a decided yellow, are extremely rare, so that we must conclude that such stones offered greater resistance to fracture than did the colourless diamonds.
Rafal Swiecki, geological engineer email contact
This document is in the public domain.