DIAMOND: OCCURRENCE IN SOUTH AFRICA 209 



crystals, of which these fragments are part, probably varied in size from 3 to 500 carats ; 

 large cleavage fragments are known as " cleavages," while fragments weighing less than a 

 carat are referred to as " splints." It is a remarkable fact that these cleavage fragments are 

 nearly always white, that is colourless, or at least very faintly coloured ; fragments of a dark 

 colour, or of a decided yellow, are extremely rare, so that we must conclude that such stones 

 offered greater resistance to fracture than did the colourless diamonds. 



The crystalline form is on the whole very regular, and the edges and corners never 

 show signs of having been water-worn except of course in stones from the river diggings. 

 Octahedra with curved and grooved edges (Fig. 31, m and o) are very frequent, while 

 rhombic dodecahedra with curved faces (Fig. 31 c), and with singly or doubly nicked faces 

 (Fig. 31 d) are rare. Crystals of this kind, when not unduly distorted, are greatly prized, 

 especially the octahedra, for it is possible to give such stones the desired brilliant form with 

 very little preliminary cleaving and loss of material. Cubes (Fig. 31 a), which are specially 

 characteristic of Brazil, are practically non-existent at the Cape, extremely few diamonds 

 with this form having been found. While hemihedral forms (Fig. 31 k) are of rare 

 occurrence, twinned crystals, on the other hand, are very abundant ; the twinning takes place 

 according to the usual law, with a face of the octahedron as the twin-plane, and the 

 individuals of the twin being two octahedra (Fig. 31 g), two rhombic dodecahedra, or two 

 hexakis-octahedra (Fig. 31 h), which are much flattened in the direction of the twin-axis. 

 The external form of twin-crystals varies with the development of the individuals, and may 

 be tabular, lenticular, heart-shaped, &c. On account of their small thickness they are not 

 ' very suitable for cutting as brilliants, and are generally used for rosettes ; they are for this 

 reason less highly prized than are other forms, such as the octahedron, and command a lower 

 price. When the junction of the two individuals in flattened twin-crystals is distinctly to 

 be seen, the stones are known in the trade as " twins," while those in which the j unction is 

 less conspicuous are referred to as " macles " (mackel). 



Besides the occurrence of twinned crystals consisting of two individuals which have 

 grown together in a symmetrical manner, there occur groups consisting of two or more 

 individuals irregularly intergrown. An example of this irregular iutergrowth is furnished 

 by the spheres of bort, which have been already described and which occur very frequently 

 both here and in Brazil. The surface of these spheres is seldom quite smooth, the projecting 

 corners of the numerous small octahedra, of which the sphere is built up (Plate I., Fig. 3), 

 being the cause of the irregularities of its surface. The size of these peculiar crystal 

 aggregates is sometimes quite considerable, spheres weighing as much as 100, or even 200 

 carats, having been found. Occasionally, the centre of a sphere of bort is occupied by a 

 single large, colourless crystal, which falls out of the rough, grey shell, when the latter is 

 broken. 



The size of the Cape diamonds is extremely variable, and ranges from that of the 

 largest to that of the smallest stones yet found in any country. 



When the operation of washing is performed with sufficient care, it is possible to collect 

 numerous stones weighing no more than ^\ carat (about 7 milligrams). The improved 

 washing machinery now in use is capable of collecting stones of this small size just as easily 

 as larger specimens. Formerly these small stones were lost in the process of washing, and 

 this gave rise to the belief that diamonds less than | carat either did not occur at all, or 

 only very rarely, at the Cape. The existence, hitherto unsuspected, of large numbers of 

 microscopically small diamonds, together with particles of carbonado and of graphite, in the 

 " blue ground " has been recently demonstrated ; the occurrence together of diamond and 

 graphite is worthy of special remark. 



