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."