166 SYSTEMATIC DESCRIPTION OF PRECIOUS STONES 



common origin for both. The fact that tlie diamond itself has never been found in a 

 quartz-vein may perhaps be explained by the extreme rarity of its occurrence as compared 

 with that of other minerals. The clays in which the precious stone lies, are decomposition 

 products of the rocks which were originally penetrated by the quartz-veins, and which, like 

 the surrounding schistose rocks, have been reduced to their present state of disintegration 

 by exposure to the action of weathering agencies. 



The deposit at Cocaes, near Ouro Preto, appears to be very similar to that at 

 Sao Joao which we have been considering. The diamonds here occur at a height of 

 1100 feet above sea-level, on a plateau of itacolumite overlying mica-schist and beneath 

 this granite-gneiss. The minerals here associated with diamond are quartz, ilmenite, 

 anatase, r utile, magnebite, haematite, martite, tourmaline, monazite, kyanite, fibrolite, 

 and gold. Of these minerals the first three in the list predominate, and quartz only 

 occurs in rounded fragments. Since the diamond and its associated minerals occur here 

 in a belt running east and west, it is possible that here also they have been derived from 

 a mineral vein similar to many carrying gold and other minerals, which traverse the district 

 of Minas Geraes in an east to west direction. 



The occurrence at Grao Mogol, in the district of Minas Novas, is of a different type 

 again. This town is situated in the extreme north of the State of Minas Geraes, on the 

 left or northern bank of the Rio Jequetinhonha and about 190 miles north-east of 

 Diamantina. As well as in the normal "gurgulho," diamonds are here found in a solid, 

 compact, conglomeratic sandstone containing much green mica, especially along the planes 

 of bedding. According to some accounts this is to be regarded as a single, isolated, 

 sandstone block of enormous size ; others, however, attribute to the diamond-bearing rock 

 an extension of 300 to 400 metres. This deposit was discovered in 1833 and was worked 

 in the thirties and forties, fragments of the sandstone being detached by means of blasting 

 powder. All the fragments of sandstone with embedded crystals of diamond, which are 

 sometimes, though rarely, to be seen in mineralogical collections, have come from this 

 locality. Such specimens are not, however, in all cases genuine, for the crystals of diamond 

 have sometimes been artificially set in the rock. 



This diamond-bearing sandstone has been, and is now by some geologists, considered 

 to be itacolumite. Those who hold this view regard the sandstone or itacolumite as the 

 original mother-rock of the diamonds now found in it, it being considered that they are as 

 truly constituents of the rock as are the quartz-grains. Later and more detailed examination 

 has, however, rendered it probable that this sandstone is not itacolumite but the more 

 recent quartzite, such as we have seen to be superimposed unconformably in the Serra 

 do Espinha^-o on the beds of itacolumite. This quartzite, although very similar in general 

 appearance to itacolumite, is yet geologically quite distinct and probably of a much later 

 date, having no doubt been formed of material derived from the weathering of the 

 diamantiferous itacolumite. Which of these two views is the more correct has not yet 

 been definitely decided. If the rock be really itacolumite, the origin of diamond at this 

 place will differ from that at Sao Joao ; if, on the other hand, it should be the later 

 quartzite, which is more probably the case, then its occurrejice here is in complete harmony 

 with that at Sao Joao, for the later-formed quartzite must of necessity contain not only 

 the constituents of itacolumite but also the minerals, including the diamond which filled 

 the veins by which it was penetrated, as is in fact the case. 



A comparison of the different diamantiferous deposits leads inevitably to the conclusion 

 that each may be regarded as typical of some one stage in the development of a single 

 process. 



