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Dry Diggings (suite)

The crystalline form is on the whole very regular, and the edges and corners never show signs of having been water-worn except of course the stones from the river diggings. Octahedra with curved and grooved edges (Fig. n and o) are very frequent, while rhombic dodecahedra with curved faces (Fig. c), and with singly or doubly nicked faces (Fig. d) are rare. Crystals of this kind, when not unduly distorted, are greatly prized, especially the octahedra, for it is possible to give such stones the desired brilliant form with very little preliminary cleaving and loss of material. Cubes (Fig. a), which are specially characteristic of Brazil, are practically non-existent at the Cape, extremely few diamonds with this form having been found. While semihedral forms (Fig. k) are of rare occurrence, twinned crystals, on the other hand, are very abundant; the twinning takes place according to the usual law, with a face of the octahedron as the twin-plane, and the individuals of the twin being two octahedra (Fig. g), two rhombic dodecahedra, or two hexakis-octahedra (Fig. h), which are much flattened in the direction of the twin-axis. The external form of twin-crystals varies with the development of the individuals, and may be tabular, lenticular, heart-shaped, etc. On account of their small thickness they are not very suitable for cutting as brilliants, and are generally used for rosettes; they are for this reason less highly prized than are other forms, such as the octahedron, and command a lower price. When the junction of the two individuals in flattened twin-crystals is distinctly to be seen, the stones are known in the trade as "twins," while those in which the junction isless conspicuous are referred to as "macles" (mackel).

Besides the occurrence of twinned crystals consisting of two individuals, which have grown together in a symmetrical manner, there occur groups consisting of two or more individuals irregularly intergrown. An example of this irregular intergrowfh is furnished by the spheres of bort, which have been already described and which occur very frequently both here and in Brazil. The surface of these spheres is seldom quite smooth, the projecting corners of the numerous small octahedra, of which the sphere is built up, being the cause of the irregularities of its surface. The size of these peculiar crystal aggregates is sometimes quite considerable, spheres weighing as much as 100, or even 200 carats, having been found. Occasionally, the centre of a sphere of bort is occupied by a single large, colourless crystal, which falls out of the rough, grey shell, when the latter is broken.

The size of the Cape diamonds is extremely variable, and ranges from that of the largest to that of the smallest stones yet found in any country.

When the operation of washing is performed with sufficient care, it is possible to collect numerous stones weighing no more than 1/32 carat (about 7 milligrams). The improved washing machinery now in use is capable of collecting stones of this small size just as easily as larger specimens. Formerly these small stones were lost in the process of washing, and this gave rise to the belief that diamonds less than 1/4 carat either did not occur at all, or only very rarely, at the Cape. The existence, hitherto unsuspected, of large numbers of microscopically small diamonds, together with particles of carbonado and of graphite, in the "blue ground" has been recently demonstrated; the occurrence together of diamond and graphite is worthy of special remark.

The most salient feature of the South African diamond-fields, as compared with those of other countries, is the prevalence of stones of large size. It will be remembered that in Brazil the discovery of a stone of 17 carats was such an event that its finder, if a slave, was rewarded with his freedom. In South Africa stones of this size occur in hundreds and in thousands; and the discovery of a stone of 100 carats causes less excitement than did the finding of a 20-carat stone in Brazil. Stones of 80 to 150 carats are of common occurrence; scarcely a day passes in which a stone between 50 and 100 carats in weight is not brought to light. Since the year 1867, when the South African deposits were discovered, the number of large stones, which have been found there, far exceeds not only the number unearthed in India in the course of a thousand years and in Brazil during a period of 170 years, but also the total production of large stones in these two countries added together. Diamonds, which weigh after cutting upwards of 75 carats, have occurred at the Cape in greater numbers than in any other known locality. While the mean size of Brazilian diamonds is scarcely one carat, the majority of South African stones are of this or larger size, excepting, of course, those stones, which are rejected as being unsuitable for cutting.

We have already mentioned that the largest diamond known, the "Excelsior," was found at the Jagersfontein mine in 1893. It is a stone of the first water, weighing 97l 3/4 carats, and is described and figured in the section dealing with large diamonds. The next largest Cape diamond is the one of 655 carats found in the same mine in 1895, which is stated to be of unusually fine quality. Another stone of 600 carats, but of poor quality, is said to have been found in this mine. A stone of 457 1/2 carats was found in one of the mines, but which one is not recorded; one almost as large, weighing 428 1/2 carats, was obtained from the De Beer's mine, while in 1892 the Kimberley mine yielded a diamond of 474 carats, from which was cut a brilliant weighing 200 carats. "The Julius Pam," a stone of 241 1/2 carats, which gave a brilliant of 120 carats, came from the Jagersfontein mine. The river diggings have also contributed a few large stones, the largest being the "Stewart" of 288 3/4 carats, which was cut as a brilliant weighing 120 carats.

Although the diamond-fields of South Africa are unique as concerns the number and size of the stones found there, the same can by no means be said of the quality of diamonds from this region. Not only are the stones frequently so dark and unpleasing in colour that they can only be applied as bort for technical purposes, but they are also very often disfigured by "clouds" and cracks, the so-called "feathers. " Moreover, this cracks, especially in stones from the Du Toit's Pan mine and from the diggings on the Vaal River, are often rendered still more conspicuous by being filled with films of limonite. The presence of enclosures of foreign matter is also common; these are usually black and resemble particles of coal, but are probably haematite or ilmenite. There are also green enclosures of a peculiar vermiform character, which, according to Cohen, are probably some compound of copper, and red enclosures of unknown nature. It is stated by Streeter that on an average only 20 per cent of Cape stones are of the first water, 15 per cent of the second, and 30 per cent of the third, the remaining 35 per cent being bort. According to Kunz, however, only 8 per cent are of the first quality, 25 per cent of the second, and 20 per cent of the third quality, the remainder being bort.

Cape diamonds show a great range of colour. Perfectly colourless or pure white to deep yellow, light to (lark brown, green, blue, orange, and red specimens have all been found. At the same time the stones may be transparent and clear or cloudy and opaque.

Diamond Geology [ 1  India  3  4  5  6  7  8  Brazil  10  11  12  13  14  15  16  17  18  19  20  Borneo  22   South Africa  24  25  26  27  28  29  30  31  32  33  34  35  36  37  38  39  40  Venezuela, Guyana  42  Australia  44  Argyle  Congo  46  47  48  49  50  51  52  53  54  55  Angola  57  58  59  Guinea  ]

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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011