DIAMOND IN INDIA
HISTORICAL REVIEW of DIAMOND GEOLOGY and MINING
Diamonds have been known longer in this country than in any other, and the most beautiful, famous and many of the largest stones were found here. Ptolemy refers to the diamond river in India; and the fact that diamonds were known to, and highly prized by, the ancient inhabitants of the country is proved by the rich adornment of the oldest temples of religion with this and other precious stones. The sacred shrines and idols show, moreover, that the art of diamond cutting has been long understood. Until the discovery of the Brazilian deposits in 1728, the supply of the whole world was derived almost entirely from the Indian sources, Borneo being at that time the only other known locality.
The occurrences of diamond in India are distributed over an extensive area of the country. C. Ritter in his Erdkunde von Asien (vol. iv, part 2, p. 343, 1836) collected together the various scattered reports concerning the diamond localities, and was the first to give a detailed and connected account. Later (1881), Professor V. Ball has given an exhaustive account, in which he has incorporated all the latest information, in the official Manual of the Geology of India (Part III. pp.1-50).
That the occurrence of diamonds in India is almost entirely confined to the eastern side of the Deccan plateau is to be gathered from the finds of the present day and from the reports of earlier times. The southern boundary of the region in which diamonds have been found is the river Penner in latitude 14º N.; from this river the diamond localities form a frequently interrupted line running northwards on the east side of the Deccan plateau, crossing the Kistna, Godavari and Mahanadi rivers, and reaching the southern tributaries of the Lower Ganges in Bengal, between the rivers Son and Khan, in latitude 25º N. Any other diamond localities outside the area just marked out are unimportant, and the reports concerning them are often uncertain. In general, many of the reported localities for diamond are doubtful, they're being no exact and reliable information respecting them, or they are simply based on the existence of old mines.
It is often supposed that all Indian diamond mines are of the greatest antiquity. In many cases the date at which the workings were commenced is not known; but the working of the most important deposits known at the present day does not date back to very remote periods, probably in all cases subsequent to the year 1000 A.D. and sometimes much later. Of a few mines it is known exactly when work began, as will be mentioned below.
Diamonds are found in India in compact sandstones and conglomerates, in the loose, incoherent weathered products of these rocks at places where they lie on the surface, and in the sands and gravels of those rivers and streams which have flowed over the diamantiferous strata or their weathered debris and have washed out the stones from their former situations.
The diamantiferous sandstone of India is of very wide distribution. It belongs to the oldest division of the sedimentary formations of the country, which usually rest directly upon the still older crystalline rocks, such as granite, gneiss, mica-schist, homblende-schist, chlorite-schist, talc-schist, and similar rocks. Fossils have not been found in these sandstones, so that it is not possible to determine exactly to which of the European formations they correspond in age; they may however be safely stated to belong to the Paleozoic period, and possibly to the Silurian division of this period.
The oldest bedded rocks known to Indian geologists are included in the Vindhyan formation. Only the lower division of this formation is represented in the Madras Presidency in southern India, and is there known as the Karnul series. In northern India, for instance in Bundelkhand, these lower beds are overlain by the later beds of the Upper Vindhyan formation. As far as is yet known, the diamantiferous sandstones of the whole of India belong to this Vindhvan formation; but while in the southern diamond districts, and probably also in the districts of the Godavari and Mahanadi rivers, they belong to the lower division, namely, the Karnul series, in northern India, for example in Bundelkhand, they belong to the upper part of the Vindhyan formation.
The Lower Vindhyan formation (namely the Karnul series) consists mainly of limestones with interbedded clay-slates, sandstones, conglomerates, and quartzites. In southern India at the base of this series of beds are beds of sandstone and conglomerates that are known as the Banaganapalli group and here constitute the diamantiferous strata. The whole of the Banaganapalli sandstones are on an average ten to twenty feet thick; they are usually coarse grained, sometimes argillaceous, at other times very compact and siliceous, and in places felspathic and ferruginous; they are dark in colour, being red, grey or brown. The pebbles of the interbedded conglomerates have been derived from the denudation of older rocks, and consist for the most part of quartzite, variously coloured hornstones and jasper, as well as compact clay-slates.
The diamonds are found in an earthy bed containing abundant pebbles; this bed is clearly and definitely marked off from other beds and is not repeated at any other horizon. The diamonds, which may themselves be regarded as pebbles since they also show signs of rounding, lie scattered singly among the other pebbles which are of the same materials as those just mentioned. This earthy bed, in which alone the diamonds are found, is of little thickness; in exceptional cases its thickness is stated to be two and a half feet, but it often measures less than a foot, and rarely exceeds this amount.
In Bundelkhand the diamond-bearing stratum belongs to the middle division of the Upper Vindhvan formation, namely the Rewah group, and is situated at the base of this group in the Panna beds. It is usually a red, ferruginous conglomerate, the pebbles of which consist, as in southern India, of quartz, variously coloured jasper, quartz-schists, sandstone, nodules of limonite, &c. The diamonds appear to bear a close relation to certain sandstone pebbles in this bed.
It is often stated, although perhaps further confirmation of the fact is needed, that in Bundelkhand diamond is sometimes found in fragments of a compact, grayish, siliceous sandstone with a peculiar glassy appearance, embedded in the stone in the same way as are the sand grains of which it is composed. These sandstone pebbles in the conglomerates of the Rewah group have been most probably derived from beds of the Lower Vindhyan formation, which, by their denudation, supplied material for the deposition of the later beds of the Upper Vindhyan formation. Thus the diamonds now found in the Upper Vindhyan formation originally belonged to the Lower Vindhyan, where, in southern India, they are still found. With the denudation of these older rocks the diamonds were set free and again deposited with the material of the younger beds, some remaining embedded in fragments of the original rock, while others became isolated and are now found among the pebbles of the conglomerate.
The diamantiferous sandstones and conglomerates are now elevated, and crop out at the surface of the ground, or are covered by younger strata. Where these strata are not so protected, as when a valley cuts across them, they will be exposed to the action of denuding agents, and will be reduced to soft incoherent sands in which the diamonds lie loosely, the whole constituting a diamantiferous sand.
Rafal Swiecki, geological engineer email contact
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