Overgrotuths 07i Diamond. 
95 
black coating are sometimes found.* The black coatings of the Kimber ley- 
area diamonds may be amorphous or crystalline diamond, or even graphite 
like the Premier ones. It may be due to a temporary increase of tempera- 
ture in past time. But a sufficiently great general increase of temperature 
is not very likely, if only for the fact that diamonds are found in intimate 
association with crystal garnet and coloured zircon. Of bort the same may 
be said ; and quite recently a lump of stewartite containing numerous 
grains of almandine garnet was found at Bultfontein. High temperatures 
would be expected to render the garnet vitreous and to blanch the zircon. 
The black incrustation is more likely to be a late corrosive sort of deposit 
on the diamond, quite independent of the process of crystallisation and not 
defining its final phase. And this view is supported by a small broken 
lump found at Koffyfontein which at first sight resembles a fragment of 
coal, but on closer examination proves to be transparent diamond thickly 
incrusted with a very hard black adamantine coat. It cannot be denied 
that the fracture preceded the incrustation. 
This Koffyfontein specimen, then, helps us to see that the black spots 
common in diamonds have not necessarily always been enclosed within 
rapidly and continuously crystallising diamonds ; and still less are these 
same spots centres of crystallisation in a metastable medium, but rather 
are incrustations on temporarily quiescent diamond surfaces analagous to 
the lime coatings dealt with above. For, suppose some black incrustation 
to form spots on a diamond crystal, then a further stage of diamond 
crystallisation would determine the black spots as inclusions like other 
minerals, such as garnet, pyrites, etc., captured in the same way. And this 
no doubt explains the frequent colour effects in their vicinity revealed by 
the polariscope. Also it helps us to see that Dutoitspan rounded yellow 
diamonds, which are much less subject to strain and are less spotted than 
most other kinds, were probably formed more rapidly in one continuous 
process. 
Haiiy, indeed, attributed the rounded form of some diamonds to rapid 
crystallisation : 
" Mais la formation du diamant ayant t-te precipitce, les faces ont subi des 
arrondissemens, comme cela arrive par rapport a une multitude de mineraux " 
(* Traite de Mineralogie,' 1801, iii, p. 290). f 
Whole stones incrusted in precisely the same way are met with now and 
* See P. A. Wao^ner, " Note on Graphite-coated Diamonds from the Premier Mine," 
' Trans. Geol. Soc. S.A.,' 1914 ; also ' The Diamond Fields of Southern Africa/ p. 143. 
t Antonio M agliabechi expressed the opinion that diamonds do not grow larger by 
lying in the earth, but that their magnitude and figure are assumed at once (see 
' Phil. Trans./ No. 311, 1707). Leuwenhoeck gives reasons for thinking differently 
(ihid., No. 324, 1709). But crystallisation, fast or slow, necessarily postulates a 
solvent. 
