Leisure hours among the gems

by Augustus C. Hamlin

In some “placers” the shapes of the crystals are perfectly regular, while in others they are greatly modified and rendered indistinct. In some mines they occur in fine octahedrons with beautifully truncated edges, but in other districts the rare form of the regular cube may abound. Every section of the diamond-bearing countries seems to have some peculiarity either in color, form, or arrangement of crystal. It is indeed true that experts, from an examination of the rough specimen, are sometimes able to detect the locality whence the diamond was obtained. The same facility may be applied to the natural crystals of other gems, but it cannot be established as a fixed or general rule. The diamond “placers,” distinct and well defined, are far more numerous on the earth’s crust than is generally believed.

A thousand plausible and often ingenious theories on the geological character of the diamond have been advanced in modern times or within a century past; and a great variety of rocks or mineral substances extending from itacolumite to xanthophyllite have been affirmed to be the parent mineral. The real matrix, or such as we believe it to be, of the diamond is the same all over the world. The associate minerals which form the conglomerate in which it is generally found may vary somewhat, but the character of the deposit is but little changed. This matrix is a secondary product, and consists of a conglomerate gravel which once abounded in remains of animal and vegetable life.

The keen eye of Buffon early detected the formation of the true gem strata; and believing that the gems were produced in these peculiar beds by the solar forces, he boldly asserted that they were formed in the superficial strata from débris of older formations, mineral, animal, and vegetable. “On ne peut leur donner d’autre origine, d’autre matrice que la terre limoneuse qui rassemblant les debris des autres matieres.”

This matrix is a well-defined conglomerate, which is found generally on elevated plateaux, and which corresponds to the bottoms of shallow lagoons or lakes of inconsiderable depth. In the earthy portion, which is composed of a variety of débris, occur the diamonds, interspersed among the quartz or rock pebbles, and in a marked and continuous layer. These formations are well known to the geologist, for they occur in almost every country on the globe.

In various parts of the earth’s structure we find solid strata of conglomerate and sandstone, which have been formed at distant and different periods of the world’s history. In other places we may observe the loose materials of the same formation awaiting transformation when exposed to the action of water highly charged with iron, lime, or silica, and we may even see the process taking place at the present day. For example, we will take the broad tracts of level country known in the south of France as the “Landes.” Here we have an excellent illustration of the formation of the gem beds, lacking, however, the precious stones. Below the surface of these plains, and generally at the depth of about three feet, a conglomerate called “allios” has formed, and is still in the process of forming. This stony layer, which is composed of quartz pebbles mixed with sand, has been cemented strongly together by the vegetable and organic matter which has trickled down from the surface of the earth during indefinite periods of time. The action of this organic débris soon renders the conglomerate quite impervious to water, and retards its passing into the sand beds or other strata below. In consequence thereof these broad tracts of level lands become stagnant lagoons of water during the rainy seasons, and all the remains of vegetable and other organic life sink down into these layers of stone, gradually filling up the interstices among the rocks and lesser pebbles.

This collection of decayed various organic substances is generally of a gray or blackish color, or may be of a rusty yellow-hue like ferruginous sandstone. Its cementing power has given a special name to the formation it has caused; and to the gold-miner it is known as hard-pan, to the gem-seeker as cascalho or “mellan.” Its peculiar hue, together with the metallic fracture of its layers, has given rise to the belief that it was composed chiefly of a ferruginous oxide. Recent analysis, however, of the “allios” has shown that this idea is partly erroneous, and that the color and the substance of the formation in reality arises from the juice and débris of plants loaded with tannin and other matter. Iron, it seems from these investigations, is present only in small quantities, and also is afforded by the secretion of vegetable life. However, the quantity of iron in the conglomerate varies considerably in different localities, but most of it seems to be acquired from the action of vegetable vitality.

These conglomerates of the Landes have been a long time in process of formation; but in other localities, like those of the Cape de Verde, to which the waves of the sea have had access, we may witness the transformation going on with rapidity. No great time is required by nature for this production, but rather the application or conjunction of certain materials exposed to the action of chemical changes and telluric forces.

It has been asserted that the diamond has been found embedded in the singular quartzite to which Count D’Eschwège gave the name of itacolumite, but we are inclined from examination of one of these specimens to think that its presence is quite accidental.

In the State of Georgia there are immense beds of itacolumite, appearing, also, here and there along the auriferous formation which extends from Virginia to central Alabama; and they afford ample fields for proof of the statement that the mineral is really the true matrix of the gem. But, after careful examination of some of these exposed rocks, we are led to regard the itacolumite as an associate mineral to the diamond, and that any farther connection with the gem is very distant. Fragments of this rock are quite often found together with quartz in the conglomerate; but we do not regard their presence as essential as that of a ferruginous oxide, which is one of the distinguishing features of all gem mines, and especially of the famous deposits of Ceylon, where the diamond is never found.

The best and most characteristic mines of India, Brazil, and Africa are situated on elevated plateaux, where there is at present but little vegetation.