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Immediately after their discovery, the yield of these mines was so abundant that other diamond districts became more or less deserted. The stones are distinguished by great purity and freedom from color, as well as by their very regular octahedral form, which obviates any necessity for preliminary cleaving, and enables them to be cut at once. For a time these mines supplied a large proportion of the total Brazilian output; they may not, however, have been as rich as they appeared, for it has been asserted that in any Cape diamonds were sent to Canavieiras in order to be put on the market as Brazilian stones and so command a higher price, just as in former times Brazilian stones were shipped to India to enter the market as Indian stones. At the present time the yield, compared with what it once was, has fallen off considerably, the deposit being now almost exhausted. The same is true to a greater or less extent for all the known diamond-fields of Brazil, and generally speaking it may be said that all are now worked to only a small extent. Brazilian diamonds, generally considered, show certain characters which are common to all diamonds, but possess other characteristics peculiar to themselves which enable an expert to recognize their Brazilian origin and sometimes even to name the actual district in which they were mined.

In size Brazilian diamonds are almost invariably small, being surpassed in this respect by Indian and especially by South African stones, many of which are above the average size. The average weight of Brazilian stones is less than 1/4 carat. Large numbers of diamonds smaller in size than the head of an ordinary pin are lost in the process of washing their size not being sufficient to repay the trouble of collecting. Stones the weight of which lies between 1/4 and 1/2 carat are frequent, those varying in weight from 1 to 5 carats are rare, and the occurrence of still larger stones most unusual. In Diamantina, when the yield was most abundant, only two or three stones of 16 to 20 carats were found yearly, and several years might elapse before the discovery of one of still larger size. Generally speaking, in early days, a lot of 10,000 Brazilian stones will contain only one stone of 20 carats, while 8,000 of them will weigh less than one carat each. From 1772 to 1830, the period during which the mines were under Government management, only eighty stones exceeding an oitava (= 17 1/2 carats) secured rightful owners; what may have been stolen is, of course, not known.

The largest Brazilian diamond is the "Star of the South", or "Southern Star", which was unearthed in the eighteen fifties at Bagagem. In its rough condition it weighed 254 ½ carats, and when cut as a brilliant 125 carats. A stone of 138 1/2 carats was found in the Rio Abaete, and one of 120 3/8 carats in the Caxocira Rica near Bagageni, while one of 107 carats was reported from Tabacos on the Rio das Velhas. Since then other stones exceeding 100 carats have been heard of.

The famous "Braganza" of the Portuguese crown jewels, a reputed diamond as large as a hen's egg and weighing 1680 carats, is probably only a pebble of transparent, colorless topaz; accurate information on the subject cannot, how-ever, for obvious reasons, be obtained from the Portuguese Government.

The crystalline form of Brazilian diamonds is by no means constant, varying in stones from different districts. Moreover, stones from different localities are not equally regular in form, those from the Cincora district, for example, being more distorted and misshapen than stones from Minas Geraes or Salobro. Generally speaking, the principal forms for all localities are the rhombic dodecahedron and the hexakis-octahedron, both having rounded faces and often deviating considerably from the ideal form (Fig. c to Fig. f.) The octahedron, which is rare, is also frequently distorted, sometimes appearing in the form of triangular plates. The predomination of cube faces (Fig. a) is especially characteristic of Brazilian crystals; such forms are very frequent here, but rare in other countries. The tetrahedron and other hemihedral forms, especially the hexakis-tetrahedron (Fig. k), are only rarely found; twinned rhombic dodecahedra (Fig. h) occur frequently; twinned octahedra (Fig. k) are, on the other hand, rare. Irregular intergrowths of diamond crystals arc frequently met with; indeed the famous "Star of the South " formed part of such an intergrowth, since its rough surface showed several impressions of smaller diamonds. Nodules of bort occur not infrequently; often they are almost perfectly spherical in form, the surface, however, being rough owing to the projection of the corners of the small octahedral crystals, which build up the radial aggregate. On the whole, about one-fourth of Brazilian stones are useless as gems; these are also described as "bort" and are applied to technical purposes.

The surface of a rough natural crystal diamond is smooth and shining, or rough, striated, and dull. Rough stones are usually opaque or translucent, but are sometimes completely transparent; in the latter case they exhibit a fine play of prismatic colors, such as is usually only apparent after cutting. The peculiar surface luster, characteristic of stones from Matto Grosso, has been previously mentioned; it is found on no other Brazilian diamonds. Diamonds penetrated iii all directions by cavities, so that their structure comes to resemble that of pumice-stone, are occasionally met with. Regularly formed depressions may sometimes be seen on the surface of a crystal; very frequently these depressions have the shape of crystals of quartz and must have been formed by the diamond resting during its growth on a quartz crystal. Diamond crystals showing evidences of contact with other minerals have been often described; the "Star of the South" is undoubtedly such a crystal, the broad under surface being very probably the area by which it was attached to the parent rock.

The color and the qualities depending on this feature vary considerably, differing in different localities. About 40 per cent of Brazilian diamonds are completely colorless and of these 25 per cent are of the purest water and the first quality, the beautiful and highly prized "blue-white" being not of very great rarity. About 30 per cent show a slight tinge of color, and though the remaining 30 per cent have a pronounced color, stones of a deep and beautiful shade are rare. Next to colorless stones, those of a dull whitish or grayish tint occur most frequently. The lighter tones of color are, seen, frequently confined to the surface of the crystal, which may be removed grinding or by the simpler process of burning, and thus the colorless heart of the crystal obtained. Such stones, and also those in which the color is confined to the edges and corners, have been found in the district of Diamantina and especially in that of the Rio Pardo and the Serra da Cincora. Deep tints of color usually permeate the whole substance of the stone. Diamonds, which are differently colored, in different parts, have also been met with. The enclosure of foreign bodies in diamonds is frequently seen; these may be dark in color or black, and sometimes resemble the moss-like markings of a moss-agate. The colors which have been observed in Brazilian stones are yellow, red, brown, green, gray, and various shades of black; blue is rare, but a few stones showing a beautiful shade of this color are said to have been found.

The general quality of Brazilian stones, it may be stated, on the whole is good, and surpasses that of Cape diamonds, which, as a rule, have a yellowish tinge. The quality of Brazilian stones very nearly approaches that of Indian diamonds, the best "blue-white" Brazilian diamonds being in no way inferior to the choicest of Indian stones.

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