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332 THE DIAMOND MINES OF SOUTH AFRICA
The average value of the diamonds per carat for eleven months was 27s. 4d.
The quality of the diamonds in the Pretoria District is poor, the
percentage of bort and rubbish being abnormally great. Valued on the
same basis, diamonds from the Pretoria District are worth only about 54
per cent of those from De Beers and Kimberley Mines.
Professor
MolengraafF considers the formation a true diamond-bearing breccia of
igneous origin, though he reports that " there is no typical vellow or
blue ground " ; but he says " this is easily explained partly by the
difference in mineralogical composition between the Elandsfontein
(Transvaal) Premier rock and the true Kimberlite and partly by the
numerous fragments of foreign rocks derived from the walls surrounding
the diamond pipes." He further states that the diamonds come " from a
peridotitic rock" which "is somewhat different from typical
Kimberlite." In other words the rock in which the Transvaal diamonds
are found resembles the blue ground of Kimberley, but is of quite a
different nature.
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