190 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



been completely removed in the course of mining operations ; in the other mines, however, 

 a little still remains to be seen. The rock at a greater depth has the character of a volcanic 

 tuft' or breccia, it is of a green or bluish-green colour, and is known as the " blue ground " 

 or " blue stuff"." Throughout the whole depth to which it has been worked it shows no 

 deviation from these characters. 



The passage of the " yellow ground " into the " blue ground " ,is as a rule abrupt, and 

 the line of division is never quite horizontal, but inclined from 5° to ] 5° to the horizon. 

 Sometimes there is an intermediate reddish layer, known as " rusty ground " which passes 

 upwards into the " yellow ground" and below into the " blue ground." Neither the "yellow 

 ground " nor the " rusty ground " is anything more than the weathered upper portions of 

 the ." blue ground ; " the latter originally filled the pipes up to the surface, but the portion 

 exposed to atmospheric influences became altered and transformed into what is now known 

 as " yellow ground." Similarly the " rusty ground " is a layer in which the alteration has 

 not proceeded as far as in the " yellow ground " ; the uppermost layer, therefore, of " blue 

 ground " marks the level below which the weathering process has not yet commenced. In 

 the early history of the mines, this change in the colour of the diamantiferous material also 

 had the effect of diminishing the value of claims, since it was feared that the " blue ground " 

 might be deficient in yield. Experience of course showed that these apprehensions were 

 groundless, for the rock at greater depths proved as rich, if not richer, than the upper levels. 

 The " blue ground," which thus fills the pipes, and from which the uppermost " yellow 

 ground " has been derived, has the appearance of dried mud, and consists of a green, or 

 dark bluish-green ground-mass, which gives its colour to the whole rock. It binds together 

 numerous fragments, larger or smaller in size, and with sharp, or in some cases rounded 

 corners, of a green or bluish-black serpentine rock. The actual material of the mud-like 

 ground-mass, and of the blocks which it cements together, is identical, the one being in a 

 finely divided condition, and the other in compact masses. These are the chief constituents 

 of the "blue ground," but it contains also numerous mineral grains as well as fragments of 

 foreign rocks in large numbers. A piece of " blue ground " of its natural colour, and 

 containing a crystal of diamond embedded in it, is depicted in Plate I., Fig. 2. 



Although the ground-mass is not very hard, it has a certain amount of toughness 

 which renders it difficult to work with a pick-axe ; it readily yields, however, to the chisel. 

 It can be scratched with the finger-nail and is somewhat greasy to the touch. The 

 qualitative chemical composition of the " blue ground " is almost identical throughout the 

 whole mass, but certain diff^erences in the quantitative composition of different portions are 

 detected in analysis. All analyses which have been made of this material record the 

 presence of silica and magnesia in varying amounts, some ferrous oxide, usually only a little 

 lime, some water and carbonic acid, and little or no alumina. The material is thus 

 essentially a mixture of hydrated magnesium silicate and calcium carbonate. 



The following is a quantitative analysis of a specimen of " blue ground " from the 

 Kimberley mine given by Professor Maskelyne and Dr. Flight : 



Per cent. 



Silica (SiOg) 39-732 



Alumina (AI2O3) ... . . 2-309 



Ferrous oxide (FeO) 9-690 



Magnesia (MgO) . . ... 24-419 



Lime (CaO) .... . . 10-162 



Carbon dioxide (COg) . . . 6-556 



Water (H^O) 7-547 



100-415 



