Popular Science Monthly/Volume 21/September 1882/Brazilian Diamonds and their Origin
By M. H. GORCEIX.
THE discovery of the diamond-beds of the Cape of Good Hope, with the extraordinary abundance of their yield, seems to have caused it to be forgotten that the empire of Brazil only a few years ago had the monopoly of that precious stone, as it still has of the finest crystals. Hopes have been entertained that the examination of the green serpentine, in which the Cape diamonds are scattered, would permit a determination of its origin and its primitive bed. No doubt, however, now exists that the magnificent crystals incased in that rock were already in existence when in its upward course from the depths of the earth it brought them along with it and left them where they are now found. Unless, then, new discoveries are made at the Cape, of beds of a different nature, it will be necessary to look to Brazil for the solution of the question of the primitive bed, the origin and the mode of formation of the diamond—a triple problem which appears, down to the present time, to exist as a challenge to all who are occupied with geology and mineralogy.
The diamond has been and still is mined at numerous points in Brazil, which are situated chiefly in the provinces of Bahia, Goyaz, Matto Grosso, Paraná, and Minas Geraes. Except in the last province and in that of Bahia, they give occasion only to the labors of isolated diamond-workers, the garimperos of Brazil, who wash the sands of the streams in large wooden bowls. The principal diggings in Minas Geraes are grouped around the city of Diamantina, which is situated almost on the meridian of Rio Janeiro, about five hundred miles from the coast. A few other districts also furnish small quantities of diamond. We shall consider especially the diamond-beds of Diamantina; and we need not then speak of the others, for they are all so nearly alike that the study of one of them is enough to give an exact idea of the others. The country here is not one that we have to discover, but to make known, and it well repays a better acquaintance. If we go to it we shall run no dangers, we need fear or seek no thrilling adventures; but we may feel as much ease in traveling as we enjoyed a few years ago in going from Florence to Bologna, or on the road to the Apennines. The security is even greater. It is said that in Mexico military convoys have been necessary to guard the trains engaged in the transportation of silver from the interior to the coast, and that even these guards did not protect them. Never has a single soldier or a single agent of the police been employed in such service in Brazil. For nearly two centuries successive caravans and numerous travelers have transported to Rio Janeiro from the most remote points of the interior fortunes in diamonds or in gold, simply packed in wooden boxes; yet we can not cite a theft that has been committed on the roads, now great highways, which were still, hardly fifty years ago, simple bridle-paths traced through virgin forests.
The rocks are at first schistose; then, in the environs of Ouro Preto, the capital of the province, appear quartzose formations, sandstones, and quartzites. These rocks constitute the peak of Itacolumi and the enormous mass of Caraca, the landmarks that guide us. After a while the white or green mica of the quartzites is replaced by spangles of oligist iron, and for several leagues the dust of the road and the pavements of the streets of the towns through which we pass are formed of the most beautiful iron minerals in the world. Quartz, mica, and oligist iron are not generally elements of a very fertile soil, but, under the action of a considerable humidity, these rocks are disintegrated and decomposed. Wherever the hand of man has not carried destruction, there is developed, under the influence of a favorable climate, one of the finest vegetations in the world.
We are now in the land of gold. The road is everywhere marked with the ancient diggings; enormous heaps of gravel on the banks of the streams indicate how considerable have been the excavations of which we see only the persisting mark-.
The rocks are always the same: mica-schists, quartzite containing mica or oligist iron, or itabaryte. The aspect of the country does not change: mountains succeed mountains, all of them rounded, gradually sloping on one side, carved into peaks on the other; and, since we follow generally the water-sheds, we have only brooks to cross, the sources of all the rivers that finally form the Rio Doce. But. after having passed the town of Serro and crossed, a few miles north of this, a chain of mountains perpendicular to the grand crest we have been following, the aspect wholly changes. Before us extends a vast plain, on which the eye hardly distinguishes a few undulations rising around the city of Diamantina. the red roofs of which are visible through a bouquet of verdure that forms a green oasis in the midst of the surrounding desert. On the right, toward the east, we may perceive a peak, the summit of which, constantly surrounded with clouds, has never been reached—less on account of its height, which is not much more than six thousand feet, than of the precipices and deep clefts which forbid approaching it. It is the peak of Itaubé. On the left is seen a less elevated mountain, formed of a single block of rounded, rough-grained quartzite, which has been given the name of Pedra Redonda. From around these two peaks rise the principal brooks that, united, form the Jequitinhonha, a stream the sands of which yield diamonds to a considerable distance below Diamantina.
The quartzose rocks predominate everywhere; the schists are seen only at rare intervals, and in their beds. But those rocks, of which quartz in grains forms the principal element, are different from those we have met in the gold-bearing region. They are more granular, only slightly micaceous; they pass into real sandstones; and the beds, generally inclined a few degrees toward the east, are less dislocated, less metamorphic than those of the schists and micaceous quartzites which they cover, and which form an islet on which is situated the city of Diamantina.
To these quartzites and sandstones are added, on the banks of the Jequitinhonha and some of its affluents, conglomerates of rounded pebbles, the horizontal beds of which occur in the same region on the banks of the Paruna, a stream emptying into the Rio das Velhas. The schists and the lower quartzites containing green mica appear around Diamantina, and in patches in the bottom of the ravines through which the Jequitinhonha flows. Quartzites, sandstones superior to the preceding, and conglomerates crowning the whole series, such are the formations, to which heaps and dikes of diorite should be added, which form the soil of this diamond-bearing basin.
The surface, in consequence of the nature of the dominant rock, is covered with a bed of white sand marked by brilliant crystals of quartz derived from the numerous veins of that substance which penetrate all the strata. Of vegetable mold there is not a trace, except in the bottoms of the ravines. In the dry season, a few kylmerias, with knotty trunks and a thick rough bark like that of the cork-oak, humble melastomas with yellow and red petals, and the opuntias, with their straight stems covered with a spiny down, are not sufficient to hide the aridity of the soil. No cultivated fields, but widely scattered houses. Everywhere, however, the ground is dug deeply, and turned over, more by the hand of man than by the action of the elements; but the only product demanded of the earth is the diamond.
The region forms a vast ellipse, the major axis of which, running from north to south, extends about fifty miles from the city of Serro to the little river Caethe Mirim, and the other axis more than twenty-five miles from the Jequitinhonha to a line parallel with the Rio das Velhas. It is, in effect, situated in the valleys of both these rivers, while a strip of it in the south, near the city of Serro, belongs to the basin of the Rio Doce. Capital differences are marked in the distribution of the diamond-bearing beds of these valleys.
In the basin of the Jequitinhonha, nearly all the water-courses, however insignificant, are or have been diamond-bearing from their sources to their mouths at that river. In the basins of the Rio das Velhas and of the Rio Doce, the streams cease to yield diamonds at a short distance from their source.
In these streams, the diamond is always accompanied by gold in flakes or in little nuggets; but while the diamond prevails in the center of the diamond-yielding region, as we move toward the east, the proportions are reversed: gold becomes more abundant; and finally, after having passed the city of Serro, it is the only precious substance contained in the sands. To this point penetrated, at the beginning of the eighteenth century, those bold adventurers, bands of whom, seeking gold for nearly a half-century previously, had crossed the mountains and reached the middle of the forest of the Sierra d'Espinhaço, dispossessing the tribes of savages whose last representatives still live miserably on the banks of the Rio Doce.
Often, down to 1729, the gold-hunters had noticed, in the bottoms of the bowls in which they washed the river-sands, little bright crystals, to which they attached no value. The brilliancy of these crystals, their hardness, and their regular form, as if shaped by the hand, had indeed attracted the attention of the miners, and many had saved them to use as counters in play; but gold alone had any value in the eyes of these adventurers. At this epoch, according to the least uncertain tradition, a monk, who had taken part in the search for diamonds in India, recognized the nature of these counters. He told his discovery to a certain Bernardo da Fonseca Lobo, who made it known in his name to the Portuguese Government. The king immediately took possession of all the lands where the presence of diamonds had been recognized, and where it could be suspected.
Bernardo received as his reward the title of royal notary, and the command of the militia of the most important city of the region. The name of the monk was forgotten. I do not believe that the name of either could have been popular at Minas, for their discovery, which threw hundreds of millions into the treasury of the kings of Portugal, was the origin of one of the most despotic rules that any country ever had to endure.
The first diamonds were found in the sands of the brooks; and these sands, or, to use the Portuguese expression which has passed into nearly all languages, cascalhos, still constitute the beds that are principally worked. But beds of an entirely different nature, situated, like mines of metals, in the midst of the strata, and of corresponding depth, have been x brought to notice in later years.
The diamond-bearing cascalhos not only occupy, or rather did occuply—for most of them have been worked—the beds of all the watercourses, but they were also placed in beds on the table-lands and in the gorges of the mountains, at levels which the waters never reach in our day, even in times of freshet. They have a peculiar appearance, that never deceives the eye of the experienced miner. At first sight, they resemble the gravels of our rivers. They are formed of rounded pebbles of various colors, and are composed of numerous species of minerals, of which I, still only at the beginning of my studies of the subject, have already recognized more than thirty. Of these, quartz, the oxides of titanium, titanic iron, tourmalines, phosphates, fibrolite, octahedric oligist iron, and magnetite, which are well known to the miners and distinguished by them under various fanciful names, are the true satellites of the diamond, its veritable train, and are with rare exceptions sure to be found with it at Diamantina. They have so intimate a connection with it, in fact, that we are justified in believing that the same formations include the primitive beds both of these minerals and of the diamond. The form of the specimens leaves no doubt as to the causes to which they owe it. They have been brought down by the waters and worn round by friction. They can not, however, have been turned into spherical balls by a simple transport of a few hundred yards. The diamond itself, the hardest of all bodies, has not escaped this action; and fragments of it are found from which every trace of crystallization has disappeared, and which are as round as marbles. Not the sands of the large streams alone, but also those of the smallest brooks, even those near their sources, present the same characteristics. The stones must, then, owe their shapes to the polishings which they have suffered by being held in the windings of the rocks and rolled around them by the eddies of the waters. While they have been thus polished off, they have produced an analogous phenomenon on the bottoms of the rivers, where they have caused the wearing out of those circular holes—the "giants' pots," the caldeiroes of the diamond-hunters, with which the beds of the streams of Diamantina are pock-marked. The sands in these holes are naturally richer than the other sands; for the lighter elements are carried away by the water, and more fragile substances than the diamond are ground to powder in them. For a hundred and fifty years the miners have considered it a piece of great good fortune to discover one of these caldeiroes; but new ones are now very seldom found. A few hundred yards above the bridge of the Diamantina road over the Jequitinhonha, the course of the water is barred by enormous blocks of diorite, between which the current has excavated subterranean passages. The river having been partly turned from its course, one may now go into one of these grottoes, which is occupied by a cascalho of extraordinary richness. The sides of the rock are as polished as the best-worked marble; the light of the torches is reflected as from a glass; and the visitor perceives at every instant cylindrical holes as regularly formed as if some skillful potter had shaped them on his wheel. The discovery of some of these caldeiroes after expensive labors, some thirty years ago, afforded sufficient return to enrich the families of the two partners who worked them. An eyewitness to the fact relates that in one of the holes under the diorite arch, when the superficial layer of sterile sand was removed, a clear mass of the precious stones was revealed, and the discoverers were able on the spot to fill their pockets with diamonds. In the Ribeirão do Inferno, a single caldeiroe of a few cubic metres' capacity furnished nearly 8,000 carats of diamonds. Such fortunes, however, are extremely rare, and can not be counted on in the regular mining.
It is easy to comprehend how powerful must have been the action of the erosion above described, the duration of which is measured by many millions of years, but is quite outside of our chronology. The greater part of the ravine through which the Jequitinhonha flows, as well as the whole groundwork of the hydrographic system of the region, is doubtless due to phenomena of upheaval, the directions of which oscillate around a north-and-south axis. The tributaries of that river have cut out channels for themselves which are now deep, close canons, with sharply cut, precipitous walls.
The almost horizontal disposition of the sandstone strata, and of the conglomerates, which form the banks of the rivers, and the terraces which indicate the successive levels occupied by the river-bottoms, leave no doubt of the correctness of this assertion. At first, consequently, the bottoms of the streams were almost at the level of the surface; the rivers overflowed their banks after light rains, and their diamond-bearing sands were spread over the table-lands and in the mountain-gorges. As the beds of the rivers were worn down deeper in the rock, overflows became more rare, and the sands were carried to only small distances from the banks. Finally, after a certain period, inundations became impossible, and the sand was deposited in the caldeiroes, the caves, and subterranean channels which the river wore out in the rocks over which it flowed. This epoch may be referred to the period which preceded our own, and which is characterized in Europe by the stone implements of human origin and use, specimens of which have been found in the diamond-bearing land.
Then, either by a rising of the coast, or, as is less probable, by a subsidence of the central plateau of Minas Geraes, the fall of the streams diminished, and, instead of continuing to excavate their beds, they began to fill them with the diamondless, shifting deposits, the formation of which is continued into our day. The sands of the table-lands and the banks of the rivers are much less rich than those of the streams; and in any case the diamonds have to be separated, by methodical washings, from the foreign substances with which they are mixed. Very rarely a simple sifting with the fingers suffices to extract the jewel. In 1824 a region was discovered in the lower part of the table-land of Diamantina where the diamonds were scattered through the soil, accompanied only by a few bits of quartz. According to the popular expression, the crystals of the precious stone could be picked out of the roots of the grass when it was pulled up. A diamond of twenty-eight carats was found on the surface of the ground. At some distance from this spot, I have myself seen, on the summit of the ridge between the valleys of the Jequitinhonha and the Rio das Velhas, diamonds in ground-cracks some inches deep, with no other companions than enormous crystals of quartz. Cavities in the rock a few metres away did not contain a trace of diamonds, although they bore identical crystals of quartz. The old miners ascribed the disposition of the minerals they sought in regular veins to the intervention of good genii. It seems as if, at Diamantina, a wicked fairy must have sown the diamonds on the ground according to its caprices; and never were caprices more whimsical and varied. . The diamond there forms only an insignificant part of the gravels, and is in most uncertain proportions.
The work of washing the diamonds is done wholly by hand. In the first operation, the sands are placed, in portions of two hundred to two hundred and fifty pounds, in a kind of hod or rectangular trough, only three sides of which are inclosed. The hods are arranged by twos, fours, or sixes, by the side of a trough of water about a foot and a half deep, so that their bottoms shall be slightly inclined toward it. A workman, standing in the trough before each hod, dashes water upon the sand in it. The clay and the very fine sands are carried away, and the first separation is made. The larger pieces remaining in the top of the sand are picked away; the diamond is to be found in the two upper thirds of the mass that is left, the lower part being nearly sterile. The washing is afterward finished in bowls a little deeper and a little more conical than those used by the gold-washers. The washer puts the sand in the bowl and fills it with water; then by whirling the bowl and shaking it up and down while the sand is floating around in it, and being careful to stir it from time to time with his hand, he determines a classification in the order of density. This work is easy if he is washing gold; for that metal is heavier than the substances with which it occurs, and always goes to the bottom.
The diamond, however, having a density about three and a half times greater than that of water, and more considerable than that of quartz and tourmaline, but less than that of the oxides of iron and titanium, its constant companions, settles in the middle layers. The washer, after several rinsings, removes the upper particles, hardly looking at them, and, when he has reached a certain level, which his skill recognizes at once, tips his bowl slightly, so as to let the water run off in a thin film, and, perceiving the glittering crystals of the diamond, picks them out with his fingers. The vigilance of the overseers must be redoubled at this stage, particularly when slaves are employed; for I know of nothing equal to the skill of the slaves in finding diamonds, except that with which they make them disappear if the vigilance of the superintendent is relaxed for an instant. I can not describe all the artifices employed, but I should remark that, since the works have become free, fraud has greatly diminished. Under the old rule it overtook half the diamonds in the gravels.
Difficulties of another kind are presented when the precious gravels are situated in the beds of the rivers. Since the channels have been deeply cut in the rock, and are bordered by steep cliffs, it is impossible to construct lateral sluices for turning the water away from them, except at an expense which even the magnificent return that is anticipated will not justify. A quicker and more simple process has been devised. The river is dammed, and a flume is made of planks to carry the water from the dam to some three or four hundred yards below. The intervening space between the dam and the end of the flume, in which the precious gravels of the caldeiroes are supposed to be situated, is thus left free from running water, while the water which stands upon it and that which reaches it by infiltration are removed by pumps worked by water-wheels. The work must be done quickly when this system is employed, for there is no time to lose. The river is generally docile enough during the dry season, from May to October; but, if only a slight storm comes on, it is transformed at once into a torrent that nothing can resist, and which carries off dams, wheels, and viaduct. This is what happens, as a rule, fifty times out of a hundred. I know of diamond-seekers who have recommenced for three or four years in succession the same works, to have them every time destroyed under their eyes by sudden freshets. Then a fortunate season has amply recompensed them for all that they had lost. Others have just had time to work for a few days in the rich beds, a few cubic yards of which have yielded them hundreds of carats of diamonds; but how many have exhausted all their resources without reaching this promised land! Nevertheless, no machines, or barrows, or inclined planes are employed, notwithstanding the enormous force furnished by the fall of water in the race is at the disposal of the operator; only picks, shovels, levers to raise the rocks, and, for means of transportation, laborers, who carry on their heads large wooden troughs, which others fill with sand and stones. Nothing can be more picturesque than these great trenches, where crowds of negroes are moving about like ants in an ant-hill, running in gangs to receive their loads, and carrying them away in groups, while they intone songs, which are almost always in the language of the African coast. The excavation becomes deeper and more sinuous; groups of men, like clusters of bees, hang, by the aid of the most primitive of ladders, to the rock walls, and the work goes on with a feverish ardor. From time to time, the overseers probe the sand with long iron rods. Great is the rejoicing when the existence of the diamond-bearing cascalho beneath the sterile sands is revealed by a peculiar sound which all the miners know. As great, also, is the disappointment when, as often happens, the probe strikes the rock without meeting the diamonds—they having been already taken away by the miners of the previous century, all traces of whose former presence have been destroyed by the river. This is another risk that the diamond-seekers have to run.
These gravel-beds are not the only treasure-bearers in the diamond yielding region. The mineral elements of which they are composed have been washed by the waters from more ancient rocks. Now, what are these rocks? Do they still exist, or have they been wholly destroyed? To answer these questions, I have carefully endeavored to determine the group of minerals which I have called the satellites of the diamond, persuaded that, wherever their primitive bed should be found, there would also be met that of the diamond. Now, all around the city of Diamantina, and for more than twenty miles west of it, the dominant rocks are quartzites with green mica, and beds of schists of the same nature and the same age as those of the auriferous formations. They are traversed by numerous veins of quartz containing oxides of iron, titanium, and tourmalines, the satellites of the diamond in the river-gravels. The origin of the latter is evidently due to the destruction of these rocks by the action of the waters; and we may, therefore, conclude that they ought to contain the primitive bed of the diamond.
The study of the geographical distribution of the diamond-bearing streams leads us to the same conclusion. All the streams, the sands of which have been found to be richest in diamonds, depart from this zone.
These deductions are confirmed also by the two facts of the discovery of the diamond in place in the sandstones with green mica, two hundred miles from Diamantina, and the discovery of clay-beds, formed from the decomposition of the schists intercalated in the quartzites, twenty miles west of the same city, where rise two rivers, the Rio Pardo and the Caethe Mirim, celebrated in the annals of the miners for their richness. The idea that the Brazilian diamonds were found only in alluvial deposits was so firmly rooted that at first no one attached importance to these discoveries. I was myself incredulous respecting them till I was able to verify with my own eyes the existence of the diamond in the rocks in place. I distinguished three formations: one white, with considerable quantities of crystals of quartz; a second gray, composed almost entirely of oxide of iron; the third, the strongest, of mottled clay, with considerable quantities of the same crystals, of rutile and oligist iron, which I have already pointed out as occurring in the river-gravels. All the formations are strongly inclined toward the east, and are intercalated with micaceous quartzites, the turns of which they follow; and were, therefore, formed at the same time with them in remote geological epochs, which, in consequence of the total absence of fossil remains, can not be precisely identified. A comparison of them with other strata of the São Francisco Valley, which are characterized by the presence of palæozoic corals, permits me to affirm that they certainly ascend as far back as the Silurian period. Other travelers, previously to myself, have mentioned this formation, and my friend the geologist Dorville-Derby, who supports my view, has carefully described it. The washing of these clays is performed in the same manner as that of the gravels, and I have myself extracted diamonds from them. In the washing, the minerals which we found rolled and rounded in the river-sands, and presenting an entirely different aspect, came out in perfect crystals, without a trace of wearing. They are the same satellites of the diamond, but still in their primitive bed. The diamonds also of São João da Chapada are characteristic: like the crystals of oxide of iron, their angles are whole; their wrinkled faces and their uniform color have suffered no modification by friction. Should not the same conclusion be adopted for the diamond, and may we not assume that it also is here found where it was formed? It is true that I have not found diamonds actually in the little veins of quartz that traverse these beds, nor in the schists, the decomposition of which has produced the diamond-bearing clays with these same crystals of oligist iron and rutile. But hardly one diamond exists to a million crystals of oligist iron; more than thirty thousand pounds of clay have furnished only ten little diamonds weighing about a carat. It would have been a rare chance to perceive, even with a strong glass, one of these stones, no larger than the head of a pin, in the midst of an enormous mass of sterile substances. Objections may indeed be made to the views which I have presented. The formations in which the diamond is found have not always been in the condition in which we now see them. They are not eruptive rocks, that have come already formed from the center of the earth, but have originated from the destruction of more ancient formations, and have undergone metamorphic action under the influence of which new crystalline elements have been formed within them, and they have assumed the aspect they now present. Why, it may be asked, may not the diamond also have been derived from these primitive formations? And then the problem, instead of having been resolved, is. only put back another step.
I may reply to this by asking if the processes of trituration effective enough to reduce crystals of feldspar and quartz to mud and sand would not also have modified the diamonds by smoothing their angles and wearing away their surface; especially, since we have seen that much lighter rubbings have been enough to produce such effects in the streams? I could likewise answer other objections that my mind suggests; and I believe I have shown that the diamond of the alluvial formations of Diamantina comes, like the iron and titanic oxides, the tourmalines and the phosphates, its faithful companions, from the destruction of the quartz-veins intercalated into the palæozoic rocks of the region. The constant association of this precious stone with those minerals causes me to believe that it has been brought, like them, from the depths of the earth in the condition of a volatile compound, and that it owes its crystallization to a dissociation produced under the action of heat and a considerable pressure. Now, what is this volatile compound?
As to the oxides of iron and titanium, together with what takes place in volcanoes, the synthetic experiments of M. Daubrée leave no doubt that they have come up as chlorides and fluorides. May the case not be the same with the diamond? Its presence in the midst of a crystal of anatase lends support to this hypothesis. This is, it is true, only an hypothesis; but it is an hypothesis based on the observation of phenomena in which I have taken analogy for my guide. Far be it from me to assume that I have resolved the problem. I shall be well satisfied if I shall ever be able to raise the corner of a veil which more fortunate and more skilled men than myself will, I am convinced, eventually remove completely.
It is very difficult to estimate the quantity of diamonds furnished by Brazil. Between 1772 and 1793 the royal treasurer received 877,717 carats of diamonds, or about 38,000 carats a year. At least as many more were stolen or smuggled away. This would make the annual production in round numbers about 80,000 carats; if we assume this average for a total period of a hundred and fifty years, we reach the figure of 12,000,000 carats, or nearly 2,400 kilogrammes (6,000 pounds), or a volume of seven or eight hundred quarts. It is impossible to calculate even approximately the total value of these stones. Generally, the diamonds of Brazil are small; stones of fifteen or twenty carats are rare; and the Star of the South, a stone which was found in the western part of the province, at Bagagem, is the only one of them that calls for a special mention. This diamond weighed, in the rough, 254·5 carats, and, after being cut, 125 carats. Finds like this are very rare; and I know of miners who have washed and washed over the cascalhos of Bagagem for twenty years, without having found a second Star of the South, or even a single diamond of value. They are still, however, far from giving up the search. The total production of Brazil in 1880 hardly exceeded sixteen kilogrammes, or forty pounds (about 80,000 carats). In the same time, the mines of the Cape yielded 2,000,000 carats. The Brazilian diamonds, however, have a very marked superiority in luster and beauty, so great that they have often been taken for Indian brilliants.