DIAMOND: SIZE OF CRYSTALS 125 



Striations due to the same cause may also be present on the faces of the hexakis-octahedron, 

 where, as before, they are parallel to one or other of the octahedral faces. 



All these twin-groupings are quite regular and conform to certain definite crystallo- 

 graphic laws. Other intergrowths of two or more diamond crystals may be met with, in 

 which the grouping is irregular and accidental, and cannot be referi-ed to any general rule, 

 the relative positions of individual ci-ystals being determined by chance. In such inter- 

 growths may be found small crystals growing singly on a larger one, or several crystals of 

 more or less equal size may be united in an irregular group. Such groups are unsuitable for 

 cutting as gems and are usually devoted to technical purposes ; the same is true to a certain 

 extent in the case of the twinned crystals above described. Irregular groupings of diamond 

 crystals may, in a crystallographic sense, be referred to as bort ; in the technical sense, 

 however, the term bort includes all stones which, from some reason or another, are unfit for 

 use as gems ; and this term is even applied to simple crystals disfigured by some serious 

 fault, such as imperfect transparency, bad colour, &c. 



Bort occurs in a peculiar spherical form, being built up of a large number of small 

 crystals radially arranged, so that the whole group takes the shape of a more or less perfect 

 sphere (Plate I., Fig. 3). Numerous small points project from the surface of the sphere, 

 these being the corners of the individual crystals which form the group. These spheres of 

 bort are found in all diamond mines to the extent of from two to ten per cent, of the total 

 output. Not infrequently only the outer shell of the sphere has the radially fibrous 

 character just described, the central portion being occupied by a large, regularly -formed 

 single crystal, which is usually so loosely attached to the radially crystalline shell that it 

 falls out when the latter is broken. 



Massive diamond with a granular crystalline structure and a black colour is known as 

 carbonado or " carbonate " (Plate I., Fig. 4). Since it is applied to technical purposes 

 only, it may be regarded in this sense as bort. It is found almost exclusively in the State of 

 Bahia in Brazil ; its characters will be further described when the occurrence of diamonds at 

 this locality is under consideration. 



Size of diamond crystals. — The size of diamond crystals varies between somewhat 

 wide limits. The smallest which come into the market sometimes measure less than a 

 millimetre in diameter, but still smaller specimens occur in nature. Small stones, measuring 

 not more than one-quarter or one-third of a millimetre along the edge may be separated 

 from a parcel of Brazilian diamonds by sifting with a sieve of fine mesh; the majority of 

 these are octahedra, while cubes and rhombic dodecahedra are but rarely present. The 

 faces of these very small crystals have the same surface characters as those of the larger 

 crystals. By carefully washing for diamonds on the Cape diamond-fields, it is possible to 

 obtain many stones very much smaller than those which usually come into the market, some 

 indeed weighing no more than -^ carat. In the method of washing formerly practised at the 

 Cape and also in Brazil, a large number of the smallest diamonds were lost, their value not 

 being sufficient to justify a special collection of them ; the improved washing machinery now 

 in use is, however, capable of saving all the stones however small. 



Stones of microscopic dimensions have only recently been observed ; previous statements 

 of supposed occurrences, such, for example, as their presence in the xanthophyllite of Zlatoust 

 in the Urals, being based on errors of determination. Microscopic diamonds have now been 

 observed in large numbers in the diamond-bearing rock of the Cape, and there is no reason 

 to doubt that they are present in other diamantiferous deposits. 



Smaller diamonds occur in larger number ; larger stones are more limited in number • 

 while very large specimens are so extremely rare and valuable that they are known by 



