A Contribution to the Study of the Biamond Made. 
163- 
rounding arises solelj from the per sal turn curtailment of the areas of 
superimposed grained laminae on the rhombs of the dodecahedron, as can be 
easily seen under magnification : Imagine a number of tiny parallel rivulets, 
some a little stronorer than others, of viscous matter to run from the middle 
each way nearly to the edge of a rhombic plane and to solidify ; then a 
second lot to overflow them in the same direction though not so far ; then a 
third lot, and a fourth, and so on, each lot in succession having a weaker 
driving **head." In the end we shall have a somewhat irregular-terraced 
sulcate elevation, rounded if the " head " has diminished at an increasing rate, 
sloping uniformly upwards to a ridge if the " head " has diminished 
uniformly. And this is about what the rippled, or sulcate surface of a 
typical Dutoitspan yellow dodecahedron looks like under the microscope. 
The rivulets here are actually due to the exposed grain of the diamond ; the 
rounded elevation is that of the rounded dodecahedron ; the uniform rise to 
a ridge is that of the tetrahexahedron, 
Brewster seems to have been the first to detect the internal grain of a 
diamond. He noticed that the flat surface of a certain plano-convex lens 
of diamond was covered with minute parallel bands, and he concluded, not 
quite correctly, that **all the bands were tiie edges of veins or laminae whose 
visible terminations were inclined at different inclinations not exceeding two 
or three seconds [of arc] to the general surface." He added that had this 
surface been an original face of the crystal there would have been nothing 
surpi.-] ng in its structure" ('Phil. Trans.,' 1841). If, however, my 
argument above is sound, then the plane face of Brewster's lens must have 
been cut parallel to a face of the rhombic dodecahedron. Parallel bands, 
would not have been seen on a plane cut in any other direction.* 
The term " grain " is used in the diamond-cutting industry, yet not 
quite in the same sense as here. Eg. Cattelle (' The Diamond,' 1911) says,. 
** Cut with or against the grain of a diamond, and the wheel makes little 
impression; it must be cut across the grain" (p. 114). Again, "Imper- 
ceptible as it is to an inexperienced eye, diamonds have a grain along which 
they can be split as wood is split, only much more evenly and exactly. 
This grain is parallel with the faces of the octahedra" (p. 126). Mineralo- 
gists have tried to say much the same of crystals in general in less homely 
language. Rutley (p. 41), e.g., says that "In the plane of cleavage the 
molecules composing the mineral are closely packed together, whilst at 
right-angles to this plane the packing is not so close. This last direction is, 
* Evidently Brewster's lens could not have been polished up to the vitreous stage 
so as to have acquired the " flowed layer of amorphous phase " which Beilby has 
suggested may be produced by purely mechanical means on the hardest crystal. 
Occasional dodecahedra from De Beers and Koffyfontein have an amazingly fine 
natural polish. Possibly their surfaces are in the vitreous stage. Boutan Le 
Diamant/ 1886, p. 37) ascribes the bands seen on Brewster's lens to multiple macling.. 
I hope to return to this matter again in a future paper. 
