[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.