Leisure hours among the gems

by Augustus C. Hamlin

Claussen published in the Bulletins of the Academy of Sciences and Belles-lettres at Brussels, in 1841, an interesting account of his observations while searching for a matrix of the diamond.

He affirms that the gems are found at the mines of Grammagon in beds of a soft sandstone, which he calls a psammite sandstone, and which resembles the itacolumite, which is much harder. He also describes several specimens in which the gems are embedded in the sandstone, but admits that they are not very common. The same writer mentions instances where they are said to have occurred between plates of mica like the flattened garnets. Furthermore, he states that the crystals found in the itacolumite are rounded octahedrons and those found in psammite sandstone are perfect octahedrons.

Claussen, although he believed the itacolumite to be the matrix of the gem, was unable to explain its total absence in places where the itacolumite was greatly developed. He was also forced, when tracing the origin of the cascalho, to admit the existence of a secondary itacolumite posterior to the transition formation.

It is interesting as well as perplexing to follow the multitude of views expressed by mineralogists when attempting to explain the formation of the diamond. Most of them are determined to give the gem an ancient origin, and insist upon the action of plutonic forces upon dioritic veins. Humboldt maintained that the gems of the Ural Mountains had a geological relation to the carboniferous dolomite of Adolfskoi as well as to augitic porphyry. But Verneuil and Murchison, examining the mines, found the alluvia which contained the diamonds had no carbon; therefore the hypothesis was incorrect, and the matrix of the stone must be sought in another direction. The mines of Brazil have been examined during a century past by a number of geologists and amateurs like Mawe, Martins, St. Hilaire, Claussen, Eschwège, Burton, Hartt, and others; and to their works we must refer the reader for extended descriptions of the geological features of the country and the peculiarities of the gem mines.

In the recent exploration of the diamond fields by Professor Hartt, the Professor decidedly opposes the views of Claussen by saying, “I do not believe that the diamond ever occurs in the true palaeozoic itacolumite in Brazil, but that it is derived from the tertiary sandstones.” After casual examination of the diamond-bearing sands of the mines in Bahia, he is also led to believe that they have resulted from the disintegration of Chapada sandstones; and he regrets that they have never been critically examined, for he thinks that the mystery of the origin of the diamond may be solved from their study.

However, from the multitude of hypotheses to which the study of the subject has given rise, we find nothing to shake our confidence in the belief of the formation of the diamond in the secondary gravel beds where they are now found.

Shortly after the opening of the Bahia mines, black, brown, and even clay-colored pebbles were found associated with the transparent diamonds in the cascalho. These pebbles were of various sizes, generally quite small, but sometimes appearing in masses as large as one thousand karats. Their nature was not at first recognized, and they were thrown aside with all other stones of little or no value. Finally a quantity was gathered and sent to a merchant in Paris, where they were seen by Count de Douhet. The Count in 1867 presented a notice of them to the Academy of Sciences and pronounced them to be massive carbon, and a variety of the diamond. The exact localities in Brazil where it occurs we are unable to describe, but believe them to be situated in the Province of Bahia. As to the quantity gathered we are also unable to give a definite opinion, but have reason to think that it is quite limited; and, moreover, we have yet to learn that it occurs in any other diamond mines in the world.

The color of the carbon, or carbonado, as it is called by the Brazilians, is generally black, but it may be light-brown or of a greenish gray color, when diluted with clay. It is always opaque, but is not always compact, being sometimes quite porous, like pumice-stone. It never crystallizes, but generally appears in angular pieces in lumps or concretionary masses whose specific gravity is 3 to 3.4, while that of the transparent diamond is 3.5.

The black and perfectly crystallized diamond, which is very rare, is not to be confounded with this variety.

The hardness of the carbon is equal to that of the transparent diamond, and probably some of the purest and most compact specimens are harder even than the limpid variety; for the black gems are generally harder than the light-colored, and we have for instances the deep-blue sapphire, the black tourmaline, etc.