A Treatise on Diamonds, and Precious Stones/Chapter 1/Section 5
SECT. V.
Natural History, Localities, &c.
The places where Diamonds have been found in modern times are—the central and southern parts of India, the peninsula of Malacca, the Island of Borneo,and some mountainous districts in the kingdom of Brazil, more particularly Serro do Frio. In the latter place, which is peculiarly celebrated, they are found in rivers or rivulets, in the banks adjoining water courses, and in ravines. The Diamond is invariably found mixed with the alluvial soil. This soil constitutes a thin stratum of gravel, resting upon Granite. On the banks and in ravines, the stratum is covered with vegetable earth more or less thick. If quartz pebbles, clay slate, brown [ron Ore, and Iron Sand constitute the alluvium[1], they are considered good indications. In one part of the Diamond District the cascalho forms a solid breccia, cementation having taken place from the oxidation of the iron: in this mass Diamonds and gold are sometimes found enveloped[2].
To explain in a more particular manner the general locality of the Diamond, I will suppose a valley to be formed by mountains of granite; through which runsa rivulet, having on its margin gradually rising hillocks or plains, more or less distant from the base of the mountains. It is by analogy that the presence of the precious gem in such a situation is to be inferred, and hence a little geological experience is highly necessary. Diamonds having been found in ravines, formed by such mountains it is fair to presume that they may. occur in others of a similar character. For ascertaining this, the experiment is easy, and demands no expenditure of capital for excavating ground either by labor or machinery. The mining for Diamonds is a much more simple operation. The mode of proceeding would be as follows: I should endeavour to procure some of the alluvium from the bottom of one of the deepest places in the rivulet; or if any little sand-bank appeared dry, I should remove the sand and mud from the surface, and scrape the gravel up, continuing this operation until I came to the solid rock, which is seldom more than two or three feet below the surface, and frequently only a few inches. I should begin the examination by putting a shovel-full of it into a bowl, commonly called by the miners a gamella, which is about 18 inches in diameter, and 10 deep, of a conical form, tapering to a point; having filled up the vessel with water, I should agitate it with my hand in all directions, and, when the water became muddy, pour it off and add more to it, holding the gamella with my left-hand, and stirring the gravel with my right, repeating the change of the water, until the earthy particles were dissolved, and the pebbles clean washed. During this agitation, from the peculiar form of the vessel, the heaviest substances in the cascalho would have sunk lowest and remain undisturbed, the surface being in a tenfold degree more exposed to this sort of trituration than the bottom.— Having poured off the clear water and thrown away the stones from the surface, the remainder would require to be carefully examined; the diamonds, if any, being heavier than the pebbles, would be found at the bottom, and may easily be discovered through the medium of water, which gives them an additional lustre, so that they cannot be mistaken. They are never found with any crust upon them, but their exterior being often rough, they have been many times improperly so represented[3],
If after continuing the examination for three or four days nothing of the kind was found, the place would be abandoned, without having occasioned five shillings' expense. If any of the precious gems presented themselves; methods would be taken to wash the gravel of the rivulets, and the banks, to the very rocks, on an extensive scale. It was by the above mentioned process, that the adventurous Portugueze, after penetrating into the interior, where they had to contend with the Anthropophagi, discovered their diamond and gold mines.
Diamonds of the smallest size, of which the weight does not exceed the tenth of a carat, and even the fifth of a grain, are the most abundant[4].
In the places where this gem is found, it appears quite out of its natural situation; nor has it yet been discovered in what substance it was originally formed.
It does not require much geological information to determine, with tolerable certainty, whether alluvial soil contain gold and diamonds; and after having acquired a little experience, any one would attain a tact that might enable him to distinguish at once, a bolsa of Indian diamonds from those of Borneo, with little risk of mistake. Nay, I have frequently discriminated Diamonds found in one part of Brazil from those in another; but the distinctive characteristics are too subtle to admit of technical description, and may yet be said to be entirely. unknown to the commercial dealers in precious stones.