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DIAMOND IN BRAZIL

Especially rich finds were made in the year 1844 in the Serra da Cincora (Sincora). This range is situated in longitude about 41º W. of Greenwich, and extends from southwest to northeast between latitudes 12º 15' and 13º 15' S. It forms the southeastern spur of the Serra da Chapada, with which it is connected at its southern end; it separates the basin of The Rio de Sao Francisco from that of the Rio Paraguassili, and constitutes the collecting-ground of these rivers. This range, the Serra da Cincora, and that of the Serra da Grao Mogol in Minas Geraes, closely resemble each other, both are rugged and inhospitable, and it is highly probable that the Serra da Cincora consists of itacolumite, although the neighboring heights are built up of granite and gneiss.

The discovery of diamonds here was due to the observation of a slave, a native of the diamond district of Minas Geraes, who, while engaged in minding cattle, was so struck with the similarity of the soil to that of his home that he began searching for diamonds, and before long collected 700 carats. Scarcely had this find become known, when eager searchers flocked in thousands to the place. According to some accounts, as many as 25,000 people had settled in the neighborhood in the following year, other estimates however place the number at from 12,000 to 14,000. Many of these fresh arrivals came from the Serra da Chapada and the Serra do Assuaria, where, in consequence of the stream of emigration, diamond-mining was almost entirely abandoned; the majority, however, were workers from Minas Geraes, where the yield of diamonds had long been gradually diminishing.

The yield of the newly discovered fields was very rich and raised the ever-sinking diamond production of Brazil to its former high level. It is said that during the most productive period the daily yield averaged 1,450 carats; soon, however, the yield began to decrease and the number of workers fell to 5,000 or 6,000. Up to the year 1849 the total output of diamonds of this district was 932,400 carats, and this immense production had lowered the prices of the stone fifty per cent. According to the estimates of diamond merchants, Bahia produced in the year 1858 54,000 carats, while from Diamantina came only 36,000 carats.

The occurrence of diamond in the Serra da Cincora is confined to the alluvial deposits of the rivers. According to J. J. von Tschudi, who quotes the statement of the traveler V. von Helmreichen, the first discovery was made on the banks of the Macule, a small tributary on the right bank of the Paraguassu. Here, besides a few small villages, there sprang up in consequence of the finds the principal town of the district, Santa Isabel de Paraguassu (also known as Coinercio), lying about 190 miles to the west of the town of Bahia. Later, diamonds were discovered at a distance of forty-five miles from Santa Isabel. The principal place to the north of Santa Isabel is Leñoes, in the neighborhood of which is Monte Vereno, a well-known diamond locality, where the diamond sands consist largely of fragments of itacolumite. Other important localities are Andrahy, Palmeiros, San Antonio, and San Ignacio.

The washings on the west side of the Serra have been poor; a considerable number of diamonds were, however, obtained from the Macule itself and from those parts of the Paraguassu and Andrahy rivers which cut through the Serra. On the latter river, the principal washings are situated on the small tributary streams of its right bank. In the bed of the Paraguassu River are depressions rich in diamonds similar to those found in the diamond rivers of Diamantina.

Diamonds from the Serra da Cincora are known as "Cincora (Sincora) stones," or as "Bahias," in order to distinguish them from the "Diamantina stones." These are considerably inferior in quality to the latter and command a much lower price. They are usually colored yellow, green, brown, or red, and almost all have an elongated, irregular form, which makes them less suitable for cutting. Diamonds of the purest water are more rare here than elsewhere in Brazil, and in size they are usually small, the large stone of 871/2 carats found at the beginning of the eighteen fifties being an exception to the general rule.

It is in the diamantiferous district of Cincora that the peculiar variety of diamond mentioned several times above, namely, the black carbonado ("carbonate"), is almost exclusively found. Although found in association with the ordinary diamond, it is so utterly unlike it in appearance that it might be taken for anything rather than diamond.

In contrast to the ordinary diamond, carbonado very rarely exhibits a crystalline form of any regularity, still the octahedron, rhombic dodecahedron, and the cube, with rough faces and rounded edges and corners, have been observed. The substance occurs much more frequently, however, in irregular rounded nodules, varying in size from that of a pea to a mass exceeding a pound in weight. The average weight of the nodules is 30 to 40 carats, but specimens weighing from 700 to 800 carats have been occasionally met with. They sometimes have the appearance of being fragments broken from a larger mass, and some show a fine striation similar to that of fibrous coal; this latter feature is believed to be due to friction between several fragments.

The surface luster of carbonado is dull and sometimes slightly greasy; the interior of the nodule is usually rather brighter and shows numerous brightly shining points. The color of the exterior always lies between dark gray and black; a fractured surface is a little lighter in color and shows a tinge of brown, violet, or red. This substance is but rarely absolutely compact, almost invariably it is more or less markedly porous, so that it is very similar in appearance to coke. When heated in water, numerous air bubbles are expelled from the spaces in the porous material. Its cohesion is usually considerable, but some samples are easily powdered. A microscopic examination of the powdered material shows it to consist of very small octahedra of ordinary diamond, usually semi-transparent, and containing many small opaque inclusions; they are nearly always of a light brown color and only very rarely water-clear. Carbonado is therefore nothing more than a finely granular, porous to compact aggregate of minute crystals of diamond, and is not, as is sometimes incorrectly stated, amorphous diamond. It differs also from the black diamonds, which occur at some localities in regular crystals built up of a uniformly compact substance. Some specimens of carbonado aggregates are penetrated in places by ordinary diamond of a lighter color, and having the usual strong luster and non-porous character. Cases are also known of the enclosure in a nodule of carbonado of a small, single, colorless crystal of diamond, the compact substance of which passes gradually into the porous substance of the carbonado shell, just as do the streaks of paler colored diamond, which sometimes penetrate the dark carbonado. The walls of the cavities in the porous carbonado are sometimes, though rarely, encrusted with minute colorless crystals of diamond.

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