218 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



Geological Survey of the Dutch East Indies, as set forth in E. Boutan's book " Le Diamant " 

 (Paris, 1886). 



The diamond-fields of Borneo fall into two well-defined groups, one in the west of the 

 island, in the district of the River Kapuas, the mouth of which lies a little below the town 

 of Pontianak, the other in the south-east of the island, not far from the town Bandjarmassin, 

 and nearly opposite the island of Laut (Fig. 42). The three portions into which the 

 western group may be divided are situated on as many different rivers, one being on the 

 river Kapuas, a little below its confluence with the Sikajam, and the other two respectively 

 on the rivers Landak and Sikajam, both tributaries of the Kapuas. The Landak deposits 

 seem to have been known since the time the Malays settled in the island, and were 

 mentioned by the Dutch mariners who first visited the coast ; indeed, from the beginning 

 the Dutch regarded the trade in diamonds in Borneo as their special monopoly. 



In the west of the island diamonds occur in beds of alluvium, in masses of debris at the 

 foot of mountains, and in the beds of streams and rivers flowing through diamantiferous 

 districts. The alluvial deposits consist of gravel, sand, and more or less ferruginous clay, 

 more rarely of conglomerate and sandstone. They are distinctly bedded, and vary in 

 thickness from 2 to 12 metres, the diamonds being confined to the lowest bed, which 

 consists of gravel. 



These ancient gravels, which themselves show little or no signs of bedding, contain 

 diamonds throughout their whole mass and are formed of more or less rounded rock-fragments. 

 They are essentially river deposits, and occur in isolated patches of small area at the foot 

 of the mountains or in the valleys, but always above the existing high-water level. The 

 rock-fragments of which these gravels are composed differ widely in kind ; white, yellow, or 

 rose quartz predominates, but there are also present hard and compact grey and black 

 quartzites, quartz-schists, clay-slates, quartz-sandstones, hornstones, hornblende, blue and 

 violet corundum, and, in sparing amount, fragments of igneous rocks, so decomposed, 

 however, that it is difficult to determine their original nature. In addition to these there 

 are also to be seen scales of white mica, grains of magnetite, a few particles of cinnabar, and 

 usually a little gold. 



It is from these gravels that the diamonds now found in the beds of streams and rivers 

 have been washed out. Both sedimentary and igneous rocks are found in situ in the 

 neighbourhood ; among the former are clay-slates and quartz-schists with quartzites of 

 Devonian age, conglomerates and clayey sandstones of much later date, probably belonging 

 to the lowest Tertiary, that is, to the Eocene age. The igneous rocks include granite, 

 diabase, gabbro, andesite, and melaphyre. 



Diamonds are only found in places where the beds of Eocene conglomerate and clayey 

 sandstone crop out at the surface, and it has been thought by C. van Schelle, a mining 

 engineer in Borneo, that it is from these beds that the diamond has been derived. In any 

 case the Devonian beds need not be considered, for no diamond has ever been found in 

 alluvial debris derived from, or resting on, Devonian strata, in spite of the fact that such 

 material has been cai-efully worked over for the gold it contains. The original mother- 

 rock and the mode of origin of the diamond are therefore here as much a mystery as 

 elsewhere, for no single crystal has ever been found in anything but what is obviously a 

 secondary situation. 



The working of the diamond-fields is in the hands of Chinese and Malays ; the former 

 work the deposits lying above high-water level, while the latter apply themselves to the 

 alluvium in the present-day water-courses, extracting the diamantiferous gravel by excavating 

 small deep pits reaching down to the solid rock, and washing the gravel in baskets. The 



