Some Controversial Notes on the Diamond. 
135 
Again, on the hypothesis of concurrent growth, how can we explain the 
twinning when one member has octahedral characteristics (? indicating 
growth) and the other dodecahedral characteristics (? indicating resorption) , 
both members being nearly of a size. 
The mutual orientation of twins does not appear to be governed by any 
law : the axes of one member may be set at any angle to those of the others. 
There is a likelihood, however, that certain alignments may be favoured 
Two macles, for example, interpenetrate by preference either nearly in the 
same plane though in opposite directions, or nearly at right angles, inter- 
mediate intersections being shunned. Even when the macles degenerate 
into bort they incline to the same rule. The McGrregor Museum at Kimberley 
has a nice specimen of this interpenetrating macled bort. 
Almost without exception there is great strain over the area where one 
of the twins cuts the other, whatever the depth of the union, due it may be 
to confused crystallisation about two sets of independent axes unsymmetri- 
cally co-ordinated. 
8. Hardness of the Diamond. 
Diamond is admittedly the hardest mineral in existence. But there is 
a good deal of misconception about the purport of the fact. Thus we are 
told (and can well believe) that there is a greater difference between the 
hardness of diamond (10) and that of corundum (9) than there is between 
the hardness of corundum and that of talc (1) ; and thence we are asked to 
infer that nothing else on earth will scratch diamond. Herbert Smith, for 
example, says that Borneo diamonds "are remarkable for excessive hardness ; 
they can only be cut with their own dust, ordinary diamond dust making no 
impression" (' Grem Stones,' p. 154, 1912). Miers says that because of 
their hardness diamonds obtained from sands or gravels are rarely, if ever 
water-worn (' Ency. Brit.,' 11 Ed., 1910). 
Now in order to be quite emphatic we will begin with the round assertion 
that there is nothing on earth that will 7iot scratch a diamond. Drops of 
water wear away the stone. Every mineral will scratch every other. It is 
only a question of time and quantity of material. Given time enough a wheel 
of gypsum, if large enough, would grind a diamond to powder. The point of 
a glazier's diamond wears out in time ; and so does carbonado used in a rock 
drill. Some years ago a large diamond was used in the De Beers workshops 
to take half an inch from the circumference of a corundum wheel fourteen 
inches in diameter. It did the work, but suffered grievous wear in the 
process. 
Thus the water-worn aspect of innumerable diamonds obtained from 
gravels is no matter for wonderment. I have seen a Vaal Eiver diamond 
worn as round and as rough as a marble, with scarce a trace of its original 
10 
