CHAPTER XXXVI

THE KIMBERLY DIAMOND FIELDS AND THEIR VOLCANIC ORIGIN

The elementary substance carbon occurs in three forms, i. e., _charcoal_, _graphite_, and the _diamond_. The commonest form of carbon is to be found in charcoal, as well as in bituminous coal, anthracite coal, and _lignite_. Graphite, also known as _plumbago_, or _black lead_, is the substance you have seen so often in the lead of pencils. The diamond, as every one knows, is the highly prized precious stone that sparkles so brightly in the light, and is so hard that it is capable of scratching almost any other substance.

Diamonds are found in various parts of the world. We are now interested in them, however, only as they occur in certain parts of the world, as in the great Kimberly diamond fields in Southern Africa.

Dr. Max Bauer in his book on precious stones says that the discovery of diamonds in South Africa was made by a traveller named O'Reilly, who, in 1867, saw a child sitting in the house of a Boer named Jacobs, playing with a shining stone. Jacob's farm was a short distance south of the Orange River near Hopetown. This stone proved to be a diamond weighing some twenty-one and three-tenths carats and was afterwards sold for $2,500. The incident led to the discovery and consequent development of the Kimberly diamond fields.

Without going into a description of the different deposits in which diamonds are found, it will suffice to say that in the Kimberly district the diamonds occur distributed through the materials that fill peculiar funnel-shaped depressions called _pipes_ which extend vertically downward to unknown depths. The rock that fills a pipe consists of an entirely different material from that in which the pipe occurs. The upper extremity of the pipe is generally slightly elevated above the general surface for a few yards. The pipes vary in diameter from twenty to 750 yards, diameters of from 200 to 300 yards being quiet common.

In 1892, the diamond-bearing material found in the pipes of the Kimberly mines had been excavated vertically downwards a distance of 1,261 feet, without any signs of its being exhausted.

Now, the materials which fill the pipe of the great Kimberly mine are practically the same in all the mines in the neighborhood. At the upper part of the pipe the materials show the action of weathering by exposure to the air. Here the ground is of a yellowish color. Below, the materials have a blue color.

According to Bauer the diamond-bearing material that fills the upper part of the pipe consists of a soft, sandy material of a light yellow color, known to diamond miners as _yellow ground_, or _yellow stuff_.

In the case of the Kimberly mine, the yellow ground has a thickness of about sixty feet. Below it the material has a blue color and is known as the _blue ground_. This latter material possesses the character of a volcanic _tuff_, which is a hardened clay. It is of a green or bluish green color and has the appearance of dried mud that holds or binds together numerous irregular, tough, and sometimes rounded fragments of a green or bluish black serpentine.

The diamonds are found near the surface in the yellow ground, as well as downwards through the blue ground. It was at one time thought that most of the diamonds existed in the yellow ground, and that they would soon disappear entirely at short distances below where the blue ground began. Under this belief some of the most valuable claims changed hands at prices far below their true value. It was soon found, however, that large and valuable stones existed in the blue ground, and, indeed, this ground has never been mined to a depth below where valuable diamonds appear.

The diamonds occur in very small quantities spread through the yellow and blue grounds. The following statement by Bauer will show this:

"A striking illustration of their sparing occurrence is furnished by the fact that in the richest part of the richest mine, namely, in the Kimberly mine, they constitute only one part in 2,000,000, or 0.00005% of the blue ground. In other mines the proportion is still lower, namely, one part in 40,000,000, a yield which corresponds to five carats per cubic yard of rock."

Of course, you will desire by this time to know the manner in which the pipes of the diamond mines of South Africa have become filled with the diamond-bearing rocks, and particularly what diamonds have to do with a book on volcanoes and earthquakes.

Dr. Emil Cohen, who has made a study of these regions, regards the pipes as volcanic vents or chimneys, and that the materials filling the pipes have been brought up from below by volcanic forces. He says:

"I consider that the diamantiferous ground is a product of volcanic action, and was probably erupted at a comparatively low temperature in the form of an ash saturated with water and comparable to the materials ejected by a mud volcano. Subsequently new minerals were formed in the mass, consequent on alterations induced in the upper part by exposure to atmospheric agencies, and in the lower by the presence of water. Each of the crater-like basins, or, perhaps, more correctly, funnels, in which alone diamonds are now found, was at one time the outlet of an active volcano which became filled up, partly with the products of eruption and partly with ejected material which fell back from the sides of the crater intermingled with various foreign substances, such as small pebbles, or organic remains of local origin, all of which became imbedded in the volcanic tuff. The substance of the tuff was probably mainly derived from deep-seated crystalline rocks, of which isolated remains are now to be found, and which are similar to those which now crop out at the surface, only at a considerable distance from the diamond fields. These crystalline rocks from which the diamonds probably took their origin, were pulverized and forced up into the pipes by the action of volcanic forces, and imbedded in this eruptive material, these diamonds either in perfect crystals or in fragments are now found."

So far as the volcanic origin of the diamonds of the Kimberly diamond fields is concerned, Cohen's theory has been generally accepted with the following modifications: that the pipes were not filled by a single volcanic eruption, but by successive eruptions, and that in the case of the Kimberly mine, the pipes contain the results of as many as fifteen successive eruptions. There has, however, been another and more important modification proposed to Cohen's theory, which is far more probable. It will be noticed that Cohen's theory regards the action of the volcanic eruption as only serving to bring fragments of a deep-seated mother rock that contained the diamonds up from below with the material that fills the pipe. Now, Prof. Carvill Lewis proposes the following very important change in Cohen's theory: that the blue ground does not consist of fragmentary material or tuff, but was forced up from below in the pipe in a molten mass and consolidated on cooling. In other words, the blue ground is filled with an ordinary igneous rock that was solidified in place in the vent or pipe.

In the great Kimberly mines the surface of the pipe is divided into numerous separate claims, each consisting of a small square lot. There are so many of these claims in the Kimberly mine that its surface is honey-combed by numerous square pits. The work is done largely by native Kaffirs employed there since the '70's. As the material was removed from the pit, the adjoining claims were separated from each other by high vertical walls.

At a later date, in order to remove the material and separate the lots, high staging provided with ropes and hauling machinery was erected. The number of these ropes is now so great that the mine has the appearance of a huge cobweb.

A very extensive series of investigations has been made at a comparatively recent date by Prof. Henri Moissan of France on various chemical products that are obtained under the influence of the high temperatures of the electric furnace. When a powerful electric current is caused to pass through a highly refractory material, that is to say, a material difficult to fuse, such as carbon, it raises it to an extremely high temperature. A still higher temperature can be obtained by causing a powerful current to flow between two carbon rods that are first brought into contact, and then gradually separated from each other, just as they are in the ordinary arc lights employed for lighting the streets of our cities. In the latter way a temperature that is estimated as high as 3,500° C. (6,332° F.), can be readily obtained. Under these very high temperatures some very curious chemical products have been obtained in electric furnaces. These furnaces consist of small chambers made of highly refractory materials closely surrounding the incandescent carbon, or the carbon voltaic arc. Among some of the most curious of these products are artificially produced diamonds.

Moissan, however, was not the first to produce diamonds artificially. As soon as Lavoisier had experimentally shown that the chemical composition of the diamond and carbon are the same, efforts were made to convert charcoal into diamonds, and Despretz, as early as 1849, by means of the combined influence of a powerful burning glass, the oxyhydrogen blowpipe, and the carbon voltaic arc obtained a very high temperature. He claims by this temperature to have been able to change carbon into a few microscopic diamonds. It is quite possible, in the light of later investigations, that Despretz may have been mistaken in his belief that he had actually produced diamonds; but whether this be so or not, he was certainly one of the pioneers in this early transformation of charcoal.

Theoretically, all that would be required in order to change the non-crystalline form of carbon into the diamond, would be to subject the carbon to a temperature sufficiently high to fuse it and then permit it slowly to crystallize. Could this be done, there should be no trouble in transforming any amount of coal into any equal amount of diamonds. But the transformation is by no means as simple as might be supposed. It is not that the temperature of the carbon cannot be raised to its point of fusion, but that as soon as a certain temperature has been reached, the carbon, instead of fusing or melting, is suddenly volatilized or turned into vapor. There is no doubt that this is done. Thousands of feet of carbon rods are volatilized every night in the arc lamps of our cities, but the trouble is that this carbon vapor so formed, when cooled, or condensed, is not converted into the exceedingly hard, clear, crystalline diamond, but into the soft, dull black graphite or plumbago.

Now the process adopted by Moissan in order to cause volatilized carbon, or carbon vapor, to condense in the form of crystalline diamonds was practically as follows: he placed pieces of pure carbon inside a very strong steel tube, such, for example, as would be formed by boring a short cylindrical hole in a piece of strong thick steel, and placing a small quantity of carbon inside the tube so formed. Closing the open end of the tube by means of a tightly fitting screw plug, he volatilized the carbon inside the tube. The steel, tube, and plug formed an electric furnace, for, as soon as he passed an electric current through it, the temperature at once became high enough to volatilize the carbon.

Under these circumstances the carbon vapor was subjected to great pressure owing to the limited space in which it was liberated. As soon as this mass of dense vapor had been formed, he seized the steel tube with a pair of furnace tongs, and plunged it below the surface of cold water in a bucket.

Of course, as the hot tube suddenly chilled, there was a great shrinking in the walls of the furnace, so that the already compressed carbon vapor was subjected to a still greater pressure which possibly liquified it. Of that, however, we cannot speak definitely. This, however, can safely be asserted, that when the tube was broken open a confused mass of small crystals was found inside, some of which, on examination with the microscope, were found to consist of small crystals of two forms of diamonds, namely, the black diamond, or carbonado, and the regular crystallized diamond.

Moissan made a great number of experiments for producing diamonds in this way, and succeeded in forming some very beautiful, though microscopic, diamonds.

What may be said to characterize especially Moissan's experiments was the comparatively great number of diamonds, so small as to be scarcely distinguishable under the microscope. The high temperature to which the materials inside the tube were exposed resulted in the production of numerous minute crystals of different minerals. In order to get rid of as many of these as possible Moissan adopted the plan of subjecting the material to the action of powerful solvents, such as sulphuric acid, aqua regia, or a mixture of sulphuric and nitric acid, and hydrofluoric acid. These acids destroyed most of the minute crystals of other minerals, but left the minute crystals of diamonds unaffected.

Now it will be observed that the theory proposed by Prof. Carvill Lewis as to the probable origin of the diamonds of the Kimberly mines bears a wonderfully close resemblance to the method adopted by Moissan for the production of artificial diamonds, since it supposes the diamonds to have been formed by the sudden cooling or chilling within the pipe of various molten materials brought up from great depths by the volcanic forces. If this be true, then besides the comparatively large crystallized and perfect diamonds found in the blue ground of the Kimberly mines, there should also be found large quantities of microscopic diamonds, just as are found in Moissan's electric furnaces, in which he produced artificial diamonds.

Moissan, considering this, obtained a specimen of the blue ground from the Kimberly diamond pipe and on subjecting it to the action of the different solvents before named, found in the mass that was left undissolved a great number of microscopic diamonds. It would seem, therefore, that there is no reasonable doubt but that the Kimberly diamond fields had their diamonds produced by the sudden chilling in the volcanic pipes of molten materials brought from great depths by the force of volcanic eruption.