Kimberley Diamonds : Especially Cleavage Diamonds. 95 
It is curious that diamonds left in the blue ground on the depositing 
floors for a number of years stick to the grease-tables less readily than those 
which are fresher from the mine. Some Bultfontein diamonds recently 
washed have this peculiarity. It is not unlikely that they have acquired a 
fine coating of lime from the soft limestone of the depositing floors. 
19. Electrical Conductivity. 
True bort, and esjiecially stewartite, is a good conductor of electricity. 
The regularly crystallised diamond is not, as is sometimes asserted, always 
a bad conductor. It is a bad conductor if its quality and texture be good, 
but if it be not highly transparent its electrical conductivity may be nearly 
as high as that of bort. Wesselton black bort, for instance, which is 
regularly crystallised though mostly in fragments, has a conductivity inter- 
mediate between transparent diamond and true bort. Some semi-trans- 
parent diamonds, greens and browns, are almost as good conductors as 
Wesselton bort Further, the conductivity will pretty often have different 
values along different directions of the same stone. As a rule, the better 
the quality the lower the conductivity, one exception being that shot bort, 
though a worse conductor than ordinary grey bort, is a better than Wesselton 
black bort. We may arrange diamonds in order of conductivity, from the 
lowest to the highest, somewhat as follows : 
Fine transparent diamond. 
Inferior transparent diamond. 
Semi-transparent or translucent diamond. 
Wesselton black bort. 
Shot bort. 
Ordinary grey bort. 
Framesite (Transvaal black bort). 
Stewartite. 
It would appear from this that impurities have as much to do with the 
electrical conductivity of diamonds as the habit has. 
20. Relative Values. 
The relative average values of diamonds exceeding about "08 carat each 
in weight, from each mine, are approximately : 
Dutoitspan. .... 100 
De Beers . . . . .86 
Kimberley . . . . .76 
Wesselton . . . . .68 
Bultfontein . . . .51 
