Overgrowths on Diamond. 
89 
their weights being great relatively to their surfaces they travel more 
slowly in a given current of water, and so can sink mechanically more 
heavily into the grease. Here it may be noted that pieces of iron and 
heavy minerals sink into the grease not because of any attribute of surface 
tension, but because the current of water flowing over the tables is 
insufficient to overcome gravitation. 
(2) Apart from that, some peculiarity of surface seemed to be involved 
whereby fractured faces were more adhesive than natural faces. On the 
other hand the sharp edges of the fractured faces can plunge mechanically 
into the grease with more facility than natural edges can. 
Under magnification, however, no characteristic difference could be 
detected either between the surfaces of the diamonds taken from the 
different tables, or between the surfaces of the non-adhesive yellow-ground 
diamonds and the surfaces of ordinary adhesive blue-ground diamonds. 
Some specific gravity tests, also, reveal no differences worth mention. 
Since only a few diamonds from the blue ground are normally lost by 
the first grease tables, it remained to examine typical specimens of blue 
and of yellow ground in order to determine whether any accountable 
peculiarities exist in the respective matrices which might be communicated 
to a diamond surface. Competent geologists are pretty unanimous that 
the yellow ground, which may go down even to 70 ft., is simply blue 
ground in a weathered condition, and contains the same minerals in due 
proportion." (W. H. Penning, ' Gold and Diamonds,' 1901, p. 4.) But 
unanimity in the case of the diamond is never evidence, and, for the most 
part, judging by the strong family likeness of the various accounts, each 
high expert quotes without question from some higher expert. At any 
rate some samples of blue ground, yellow ground and so-called " limestone " 
from the original overburden, were obtained for testing purposes from the 
Wesselton mining area. So far as could be seen the samples of yellow 
ground were typical and clean. The blue ground had other matter mixed 
with it, which had probably been scraped up from the depositing floors 
by the shovel. It could only be guessed whether the "limestone'" was 
an average sample of its kind. It was a spongy-looking stuff of rather 
low specific gravity (? altered blue ground, or dolomitic), and to casual 
inspection carried particles of yellow ground. Portions of each of the 
three kinds of ground were crushed, gently dried, weighed, treated with 
dilute acid, evaporated to dryness, and again weighed, with the following 
result : 
Weight before 
treatment. 
Weight after 
treatment. 
Blue 
Yellow . 
*' Limestone " 
10-000 
10-000 
10-000 
9-225 
8- 650 
9- 975 
