Overgrowths on Diamond. 
97 
explosion! — clouds of tiny black discrete particles. One would like to 
understand how the internal black spots are aligned with respect to the 
space lattice, as to which there is a striking suggestion in the case of a 
coated Bultfontein " rounded octahedron," within which at a depth of about 
a millimetre (as determined roughly by the focussing distance) under one 
face were three separate constellations of tiny black spots all lying in one 
octahedral plane of cleavage.*' These constellations of spots were of 
additional interest because of the gorgeous chromatic display they gave 
under the polariscope, and also for their obvious relationship to the surface 
markings, for exactly over each constellation was a particularly well-defined 
indented triangle. Could the millimetre or so of the face have been cleared 
off, then without a doubt the new face would have had much the same sort 
of coated appearance as the existing natural face had. 
I have observed the phenomenon of the superimposition of a deep 
indented triangle over a shallow-seated spot or spot area once or twice 
before, and thought that it could be explained as mere coincidence. The 
case of the above-mentioned Bultfontein stone nevertheless makes it pretty 
certain that spots may on occasion interrupt the orderly rate of crystallisa- 
tion, and hence that they are not likely to be action centres of growth 
themselves. 
It is quite on the cards that a competent study of the black incrustation 
and the black spots may yet explain a great deal that is yet unknown 
about the diamond made and so justify the use of the name. For the 
majority of macles are spotted— some in the composition plane — as it is 
not quite accurately termed, seeing that the seam is not as a rule a plane 
at all — ^others in most remarkable streaks along the grain, and yet others in 
both ways together. The streaks running along the grain — i. e. in the 
dodecahedral cleavage planes — are very common in macles, and practically 
non-existent in simple crystals. Some macles display, indeed, saving for a 
comparatively clear space in their centres, almost as much streak as 
diamond. G-ranting, for argument, that the streaks are graphite, they show 
at least that macles must have grown in a more graphitic environment than 
simple crystals did. It is not meant by this that the graphite must have 
forced the macling, for many macles are quite free from streaks and spots 
too, but that the two sorts of conditions were unlike under which macles 
and simple crystals came into being, conditions perhaps in which the pro- 
duction of graphite was more favourable than it was elsewhere. In this 
connection we may observe that the blue grounds of the Premier and 
Jagersfontein Mines are reported to yield a large percentage of macles and 
much graphite. 
Falling within the category of coated diamonds, though not in the 
* Cf. two observations of a similar kind made by Des Cloizeaux, cited by Boutan;, 
p. 55. 
