Some Controversial Notes on the Diamond. 
131 
twin liabit (a somewhat rare thing' in the Kimberlej mines),* measuring 
6"7 mm. from corner to corner, and 5'5 mm. thick. To cursory inspection 
one of its triangular faces seems to be much cracked, bufc under moderate 
magnification the simulated cracks resolve themselves into deep natural 
grooves penetrating from a half to the whole way down to the seam (i. e. 
from a quarter to half way through the stone), the sides of these natural 
grooves not probably being in actual contact anywhere. On the face of the 
stone the sides of the grooves are striated like tiie edges of octahedra com- 
monly are, or like the faces of rhombic dodecahedra from Bultfontein. It 
really seems as though the stone must have grown outwards from the seam 
in separate portions all at the same rate, keeping, that is, the overall 
dimensions to the correct spinel twin habit. The annexed picture is from a 
rough pen and ink sketch of the appearance of the stone. The thick lines 
indicate the grooves. This diamond evidently could be broken up into 
distorted fragments having pseudo-cleavage faces. 
Sometimes two diamonds appear to have formed, one ou either side of 
some foreign matter, the two together making one stone of passable shape. 
An interesting though not exceptional case is that of a Koft'yfontein 
diamond of about two carats, one half of the diamond being white and the 
other half yellow with a tinge of brown. A light tap on the diamond with 
a metal weight, and it fell into two parts along the plane dividing the two 
colours. Inside there were tw^o tiny zircons and some apophyllite. This 
was certainly not a case in whicli the diamond had formed normally about 
the zircons and split afterwards, for the crystallisation had proceeded by 
two separate intentions. Precisely similar cases of crystallisation on either 
side of flakes of diamond are met with. 
5. Groups and Clusters. 
Sir H. A. Miers observes Ency. Brit.,' 1910) that "the majority of 
minerals are found most commonly in masses which can with difficulty be 
recognised as aggregates of crystalline grains, and occur comparativelj 
seldom as distinct crystals ; but the diamond is almost always found in 
single crystals, which show no signs of previous attachment to any matrix." 
The examination of many hundred specimens prompts me to suggest that 
his statement needs some modification. It may be granted that regular 
octahedra and other tesseral forms were almost certainly formed as single 
crystals ; but the statistics given in " Kimberley Diamonds " show that 
these only make up a quarter of the whole production. Is it safe to 
dogmatise about the remaining three-quarters, even if there were no direct 
evidence '? As it happens there is a good deal of direct evidence. Leaving 
out of account bort (which may be regarded as aggregates of crystalline 
grains), and interpenetrating crystals (found by the hundred at Bultfontein), 
* Most Kimberley macles have rounded edges without re-entering corners. 
