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DIAMOND in SOUTH AFRICA

Dry Diggings (suite)

Pure white, absolutely colourless stones are very rare, still the finest blue-white diamonds, such as are found in India and Brazil, and are not altogether absent. Only about 2 per cent of the total number of stones found reaches the standard of absolute perfection, among such the most general form is that of a symmetrically developed octahedron. The "Porter Rhodes", found in the Kimberley mine on February 12, 1880, is one of the finest of Cape diamonds; it is said to weigh 150 or 160 carats, and is a stone of singular beauty. The largest diamond known, that of 97l 3/4 carats, as well as those of 655 and 209 1/4 carats respectively, all from the Jagersfontein mine, are also of this high quality. As a rule, large stones are patchy and impure or coloured yellow, often a deep shade of yellow, which greatly diminishes their value. The otherwise poor deposits of Jagersfontein and the liver diggings are remarkable for the purity and beauty of the stones found there, especially in the latter.

The majority of what are usually regarded as white Cape diamonds are in reality more or less tinged with yellow; an experienced diamond merchant, though not apparent to an unpractised eye, at once remarks this. Stones of this tint are described as being "Cape white", while others, in which the faint yellow tint is replaced by an equally faint greenish tinge, rank as " first by-water. " Although the yellowish and greenish tinge is so slight, yet it manifestly exercises a considerable influence on the lustre and refractive power of the stone. Such a stone scarcely attains to the fire and play of colour of a perfectly colourless Indian or Brazilian stone; moreover, even though cut in the best brilliant form, it will appear dusky when compared with the latter and will therefore be less highly prized.

Stones of a distinct, though pale, yellow colours are especially common; they vary in shade from a canary- or straw-yellow to a light coffee-brown. They form the majority of those Cape stones, which are suitable for cutting, and are naturally less prized than the Cape whites or others already mentioned. As a rule, these stones, the different shades of which are distinguished by the terms second by-water or off-coloured stones, pale yellow and dark yellow, are less disfigured by faults than are the colourless stones. The abundance of these pale yellow stones is a feature peculiar to the South African diamond-fields; nowhere else are they found in such numbers. Before the discovery of these deposits, stones of this colour were extremely rare and were sought after as much as are now the stones of a fine red, blue, or green colour, which are still rare. Such diamonds are referred to as "fancy stones," and are perhaps more rare at the Cape than at other localities; a representative of such "fancy stones" from the Cape is a rose-violet diamond of 16 carats. Diamonds of these beautiful colours, even when found, are invariably small. Transparent stones of a dark brown or black colour are very rare; though the qualities most highly prized in colourless diamonds are absent in such stones, vet, on account of their rarity and their application in mourning jewellery, they command a high price. Very darkly coloured or impure stones, as well as those which are cloudy and opaque, are unsuitable for cutting and are used as bort.

Another unique feature of the Cape diamond-fields is the occurrence of the peculiar "smoky stones," which have been already mentioned. These occur for the most part at Kimberley and are scarcely known elsewhere; they are distinguished by their very regular octahedral form and by the possession of a peculiar smoky-grey colour, which is either distributed uniformly or accumulated at the edges and corners of the stone, which, in the latter case, is known as a "glassy stone with smoky corners". In these diamonds there is a liability, as has been already mentioned, to fall to powder with no apparent external cause; this is certain to happen sooner or later when such a stone is once taken out of the ground, and many and various are the devices adopted by the unfortunate possessor to postpone the catastrophe at any rate, until he has prevailed on some inexperienced buyer to take the stone. Thus, immediately after it is taken from the rock, the finder will perhaps place it in his mouth, or smear it with grease; and when it must be sent on a journey it will often be placed inside a potato, this being considered the safest method of packing such stones, probably because they are thereby protected from contact with other diamonds or hard objects, the slightest scratch being sufficient to bring about the bursting of the stone. The singular behaviour of these stones is due to the existence of intense internal strains in their substance; the same phenomenon being also the cause, as we have seen, of the strong anomalous double refraction possessed by some diamonds.

The collective characters of the stones found in each mine and in each part of a mine are distinctive, but single stones of every quality occur in all mines. Thus, though it may be impossible to state the particular mine in which a single stone was found, yet an experienced Kimberley diamond merchant would have no difficulty in naming the mine, or portion of a mine, from which a parcel of stones had come, provided that the parcel formed a fair sample of the yield of that particular deposit.

The stones found in the rich Kimberley mine are usually poor in quality; and broken fragments, the latter invariably uncoloured but containing many black enclosures, are present in great abundance. A large percentage of the material yielded by this mine, especially from the north side, is unsuitable for cutting and only applicable as bort, 90 per cent of South African bort being furnished by this mine. Broken fragments are confined to a certain extent to the middle and south side of the deposit; while the north-east corner and the west side of the mine have yielded brown octahedra and "smoky stones" in great abundance; yellow diamonds, so numerous everywhere else, are here almost wholly absent. The stones found in the cast and southeast portions of the deposit closely resemble those from Du Toit's Pan mine.

From the De Beer's mine are obtained crystals of every kind and colour, the surface of which is almost invariably finely granular, glimmering, and somewhat greasy, surface characters which are met with in crystals from no other deposit. Bort is rare, but broken fragments containing black specks are abundant. Large yellow rhombic dodecahedra are very frequent, the De Beer's stones being, on the whole, remarkable for their large size, while stones from the Kimberley mine are conspicuous for their whiteness.

The diamonds found in the De Toit's Pan mine are usually well crystallized and of considerable size, yellow octahedra being often especially large. Bort, very small stones and "smoky stones," are practically absent, and crystals disfigured by black specks are seldom met with. The colour of stones from this mine is often rather dark, but the proportion of Cape white and yellow stones found is greater than elsewhere in this region. On the whole, the diamonds yielded by this mine are more beautiful than those of any other deposit in the neighbourhood of Kimberley.

The Bultfontein mine yields principally small white octahedra, much modified on their edges and usually full of faults and spots. Large stones, broken fragments, bort, and deeply coloured stones are here practically absent.

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  ]


Related links: Diamonds: Large and Famous   Properties   Geology and Mining Diamond Cutting Gem Cutting Diamond Trade  Values of diamonds
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Rafal Swiecki, geological engineer email contact

This document is in the public domain.

March, 2011