192 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



and are therefore rather inconspicuous. A complete collection can only be made from the 

 residue left after the process of diamond-washing. 



Among these minerals the most important, but not the most frequently occurring, is 

 the diamond : it is found in crystals developed regularly on all sides, and also in fragments, 

 such as would result from the breakage of larger crystals. It is remarkable, however, ihat 

 different portions of the same crystal are never found lying close together. The edges and 

 corners of the crystals are always perfectly sharp, not even the faintest trace of rounding can 

 be detected, so the stones of the dry diggings are easily distinguished from those of the 

 river diggings. A more detailed description of the special characteristics of Cape diamonds 

 will be given later, here we are concerned only with their mode of occurrence. 



The diamond is a constituent part of the agglomerate in which it is embedded, and its 

 mode of occurrence in no way differs from that of other minerals contained in the rock. 

 Each crystal or fragment of a crystal occurs alone, firmly embedded in the agglomerate, 

 from which it can be extracted only with difficulty ; its surface is usually clean, but in some 

 cases is coated with a layer of limonite (iron hydroxide) or with a calcareous film, both of 

 which are easily removable. Until recently, no diamoiid had ever been observed attached to 

 another mineral in such a way as to suggest that the two grew side by side at the same 

 time. The discovery, however, of a diamond crystal attached in this way to a garnet shows 

 that such a growth does take place, though rarely. 



Diamonds are to be found at the surface, and downwards through the "yellow ground," 

 the " rusty ground," and the " blue ground," as far as the deepest mines have yet penetrated ; 

 they do not occur, however, in equal number in all mines, nor in different portions of the 

 same mine ; numericn.l details will be given later. In the Kimberley mine, which is unique 

 in this respect, the richness of the yield increases rapidly as lower levels are reached. The 

 different col umnar divisions of each pipe vary in the number of diamonds contained, some being 

 so poor that the working of them is unprofitable, others on the contrary being just the reverse. 

 The total number of diamonds contained in a given mass of any particular column is so 

 constant that it is quite possible to calculate beforehand how many carats of stones a certain 

 amount of " blue ground " will yield. 



The presence of diamonds in the " blue ground" is of enormous economic importance ; 

 regarded as a rock constituent, however, they ai'e quite insignificant, being present in such 

 small amount that, had they been less highly prized, and of less general interest, they would 

 probably have been scarcely mentioned in a petrographical description of the rock. A 

 striking illustration of their sparing occun-ence is furnished by the fact that in the richest 

 part of the richest mine, namely the Kimberley mine, they constitute only one part in two 

 millions, or 0"00005 per cent, of the " blue ground." In other mines the proportion is still 

 lower, namely one part in forty millions, a yield which corresponds to five carats per cubic 

 yard of rock, and which can be profitably worked. When the absolute amount of diamond 

 present is so small, slight variations in this amount in diff'erent parts of the rock, though of 

 great economic importance, are of little scientific significance. 



In respect to the associated minerals of the diamond in the agglomerate, certain 

 differences exist between the various mines and between different parts of the same mine. 

 These minerals occur either in homogeneous grains of the same kind, or in small groups 

 consisting of minerals of various kinds. Those of most frequent occurrence are red garnet, 

 green enstatite, and vaalite (an altered mica), others are less widely distributed, some indeed 

 being regarded as great rarities. The most important of the minerals associated with the 

 diamond in these deposits will be now considered in order. 



The garnet is of constant occurrence, and always in relatively considerable amounts. 



