' 164 Transactions of the Royal Society of South Africa. 
therefore, a direction of least cohesion, and hence splitting or cleavage easily 
occurs along it." Also P. von Grroth (' B. A. Eeport,' 1904) tells us that 
those planes which are parallel to the greatest density of structure — what- 
ever that term may be supposed to mean precisely — are identical with the 
cleavage planes. Of course, what these authorities really mean to say is 
that the molecules are probably most closely packed in some given direction 
because a cleavage plane runs that way. By saying the other thing they 
put the cart before the horse. But it would seem that a diamond is most 
readily cleavable parallel to an octahedron face because the grain of each 
of three planary directions runs parallel to a face. It is not so easily 
cleavable parallel to the faces of the cube, because only the grain of each of 
two planary directions is parallel to a face, and it is still less easily 
cleavable parallel to a rhomb of the dodecahedron because the grain of only 
one planary direction runs that way ; so that in the last case it is only 
across the thin edge of a made that we should expect to get this sort of 
cleavage to the best advantage. All the same, it is surprising how good 
such cleavage from a simple crystal may be on occasion. Plates of cleavage 
parallel to a dodecahedral plane of symmetry are met with on the sorting 
tables in which both cleavage faces are as nearly parallel to each other as 
the faces of a portrait stone, and, moreover, are almost as natural looking 
as a face of the dodecahedron itself. Fractures parallel to a cube face are 
much less elegant as a rule. 
We may summarise the last paragraph by saying that there 'isilld-Qvee 
orders of diamond cleavage : 
First, that parallel to an octahedron face ; 
Second, that parallel to a cube face ; 
Third, that parallel to a dodecahedron face ; 
whence the cleavage of diamond is not so much a question of " density 
of structure," or concentration of molecules, as it is of array of molecules. 
It may be of interest to note here that the great Eobert Boyle failed to 
distinguish between the true grain of a diamond and the " grain " as under- 
stood by diamond cutters. He had observed the thin plates exposed on the 
broken surfaces of " New English Granats,'' " and to try whether this 
observation would hold even in the hardest Stones, I had recourse to a 
pretty big Diamond unwrought, which being plac'd in a Microscope, shew'd 
me the Commissures of the Flakes I look'd for, whose Edges were not so 
exactly dispos'd into a plain, but that some of them were very sensibly 
extant like little Ridges, but broad at the Top above the level of the rest. 
And these Parallel flakes together with their Commissures, I could in a 
somewhat large Diamond plainly enough discern even with my unassisted 
Eyes. And for further satisfaction I went to a couple of Persons, whereof 
the one was an Eminent Jeweller, and the other an Artificer, whose Trade 
was to cut and polish Diamonds, and they both assur'd me upon their 
