GEMS, NATURAL AND ARTIFICIAL 



cates, is very hard. Above the "hard bank" is the 

 softer "blue ground/' so named from its bluish color. 

 And above this the "yellow ground," greasy to the 

 touch, soft, friable, and yellowish in color. Yellow 

 ground and blue ground are supposed to be decom- 

 posed stages of the hard bank, the different colors 

 being due to the presence of different iron oxides. 



Some of these diamond-bearing veins can be worked 

 from the surface in the early stages of mining, but if 

 the work is to be carried on extensively it is necessary 

 to sink shafts and tunnel just as in other subterranean 

 mining. The yellow ground, being soft and friable, 

 may frequently be worked as soon as it is brought to 

 the surface ; and it is possible to work even the hardest 

 material by crushing. It is more economical and satis- 

 factory, however, to spread this hard material out in 

 thin layers and allow it to be acted upon by the ele- 

 ments. At Kimberley there are great fields, or "floors," 

 covering many square miles which have been spe- 

 cially prepared for this purpose. The blue ground 

 is spread over this to a depth of two and a half feet, 

 and allowed to disintegrate, the process taking from 

 a few weeks to two years. Even then it is sometimes 

 necessary to reduce the harder masses in the crushing- 

 mill. 



From the floors the material goes to the washing- 

 plant, where the heavier materials are separated from 

 the lighter ones by complicated washing and agitating 

 machinery. This heavier material containing the dia- 

 monds is passed on to machines known as pulsators, 

 which concentrate and drain the diamond- bearing 



