OCCURRENCE AND DISTRIBUTION OF DIAMOND
Whenever possible, I provide, in parentheses, today's equivalent prices in US dollars. These prices take account of inflation and are based on available historical exchange rates. The inflation rate is calculated on the assumption that 1 oz. of gold has always the same value; only due to a loss, with time, of currency value it takes more money to buy the same 1 oz. of gold.
Diamond has been found in all five continents, but not to the same extent in each. It has been longest known in Asia, where the famous old Indian deposits have probably been known and worked from the earliest times; now, however, they are almost completely exhausted. In close geographical connection with these are the deposits in Borneo, but the supply from this island, in comparison with the rich treasure of India, has always been limited. Reported discoveries of diamonds in the Malay Peninsula, where, according to one account, the famous Regent of the French crown jewels was found, in Pegu and Siani and the islands of Java, Sumatra and Celebes, are for the most part unauthenticated; and the same may be said of the reported occurrence in China (province Shan-tung), Arabia, Cambodia.
In America the famous Brazilian diamond-fields were discovered at the beginning of the eighteenth century, and have compensated for the exhaustion of the Indian mines; the mines in the States of Minas Geraes and Bahia have given the richest yield of stones. Finds have been made in another parts of the South American continent, namely in ex British Guiana; Guyana, Venezuela and Brazilian State of Roraima. Well authenticated, but of little commercial importance, is the occurrence of diamond in the United States of North America; a small number of stones having been found in the eastern States of Georgia, North Carolina, South Carolina, Kentucky, Virginia, Wisconsin, and in the western States of California and Oregon. Reported occurrences in other parts of the American continent, namely Sierra Madre in Mexico, and the gold mines of Antioquia in Colombia, South America, require confirmation.
The continent of Africa is, at the present time, by far the most important source of diamonds, which have been collected here since the late sixties in ever increasing numbers, far surpassing the yield from any other region. The exact locality of the deposits is on the Vaal River, and in the neighborhood of the town of Kimberley, both these localities being in the division of Griqualand West in the north of Cape Colony; also in the adjoining Orange River Colony, which, however, is of far less importance. Compared with the yield of the African fields, all others are insignificant, although in comparatively recent times the markets of the world were supplied from the sources which are now of such minor importance. During last 100 years, the diamond fields of the Cape are the source of 90 per cent of the stones, which come into the market. The reported occurrence of diamonds in the auriferous sands of 'the river Gumel, in the province of Constantine, in Algeria, is unauthenticated; three stones were said to have been found here in 1833, but nothing more has been heard of this reputed discovery. The statement of Dr. Cuny, an African traveler, that in the fifties a whole camel-load of diamonds was brought from Western Africa to Darfur seems rather incredible, or he was talking about diamonds from Siera Leone. Later Congo becomes an important source of mainly industrial diamonds. The presence of alluvial diamonds in sands, on seashores of Namibia, is well known.
In Europe, diamonds have been found in Russia, in the Urals in the east, and in Lapland in the west; the stones are, however, met with only in small numbers, and their importance lies in their mineralogical rarity. The occurrence in Russia is of major economical importance.
Diamonds have been found in recent times in Australia, especially in New South Wales, not altogether in inconsiderable numbers; and Australian stones are at least mentioned in the markets.
Finally we must record the interesting fact that diamond is not only a constituent of the earth's crust, but also of extra-terrestrial bodies, the presence of small stones having been reported to be found in several meteorites.
With regard to the mode of occurrence of diamonds, it is to be noted that in the majority of localities they are found in secondary deposits, such as sands and gravels. These masses of debris produced by the weathering of the original mother-rock of the diamond are usually entirely loose and incoherent; occasionally, however, as in Brazil and India, they are converted by cementing materials into firm conglomerates, breccias and sandstones. In Brazil these rock masses, like the loose sands and gravels at other places, lie on the surface, and must therefore be reckoned among the most recent deposits of the earth's crust. In India, and to a certain extent also in Brazil and North America, the diamantiferous fragmentary rocks belong to earlier geological periods, being interbedded with some of the oldest rocks, and thus representing the sands and gravels of very remote ages. When these older fragmentary rocks come to the surface, they are themselves in course of time attacked by weathering agents and supply material for new secondary deposits, from which the ordinary process of washing wins diamonds.
The nature and character of the original mother-rock, in the debris of which the diamond is now found, has nowhere been determined with the certainty and clearness that is desired, although many important steps have been made towards the solution of this problem. In the following pages we will consider in detail the facts connected with each well-established occurrence of diamond, and endeavor to determine the origin of the stone in each case so far as the available observations permit. In any case it is certain that the original mode of occurrence and the mother-rock are not the same at all localities: in some cases the mother-rock is without doubt one of the older crystalline rocks; in other cases it is highly probable that diamond originated as a secondary mineral in the rock known as itacolumite, as will be specially considered when we come to treat of the Brazilian deposits. In the South African diamond-fields, the stones are found for the most part embedded in a green serpentine-like rock; known as kimberlite, instead of in loose sands as is more usually the case. This mode of occurrence, which is not peculiar to this locality, and differs from that of all others, will be considered in detail under its appropriate heading.
The different diamantiferous deposits will be dealt with below in the following order:
Rafal Swiecki, geological engineer email contact
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