In quality the Vaal River stones are rated highest, in the degree that while in the three years ending with August, 1885, the weight of those sent away from that district was only about 1⁄125, that of the entire exports their declared value was as much as 1⁄62 of the total value exported. Of the four Kimberley and Beaconsfield mines, Dutoitspan produces the finest stones, ninety per cent being perfect. Bultfontein comes next, with its beautiful white stones, weighing from a quarter of a carat to two carats. De Beer's comes next, and Kimberley last. In absolute value of production, the order is, Dutoitspan, De Beer's, Kimberley, Bultfontein. The largest diamond ever found in Griqualand West was an irregular octahedron from Dutoitspan, slightly spotted, and of yellow color, which weighed 404 carats, or nearly three ounces. The only larger stone than this known to have ever been found was a very imperfect one, discovered near Jagersfontein, which weighed about 500 carats. Another diamond, of 352 carats, has been found at Dutoitspan. The largest stone ever found at Bultfontein weighed a little more than 150 carats. The Bultfontein diamonds, while superior in color to all others except those of Jagersfontein, are of smaller average size than those of the other three mines.
All the diamonds coming from the various South-African mines are said to have a distinct personality, by means of which experts can at once recognize stones from either of the four mines, and tell from which it came, and can again distinguish those of the Kimberlev and Beaconsfield mines from those of the river-diggings, and their testimony on this point is accepted by the courts.
All the theories by which the attempt is made to account for these mines recognize them as of volcanic origin. Their form suggests at once the crater of an extinct volcano, or the tube of a geyser. When the attempted explanation goes beyond this, the range of diverging opinions is quite wide. Of these various views, we will refer only to the observation of Sir Henry Roscoe, that the most noteworthy feature of the diamond-bearing rocks of Kimberley is the discovery in the diamond-earth of a volatile crystalline hydrocarbon, soluble in ether, which seems to confirm the hypothesis that the carboniferous shales, which are penetrated by the diamond-bearing pipes, have been the source of the carbon now found in the crystalline state in this gem. The physical structure of the ash, or incombustible portion of the stone, is of a very singular character, and has hitherto not been examined. A careful study of it may possibly throw light on the important question of the mode of formation of the diamond.
