South
African diamond fields are fully discussed in the volume of Mr. F.
Gardner Williams on " The Diamond Mines of South Africa." Here it must
suffice to state that the De Beers' and Kimberley floors, whither
the " blue ground " is conveyed, and where it is spread out to weather,
cover an area of two thousand acres. Here the blue ground is harrowed,
and, if necessary, watered. After various crushing, washing and
screening operations, a material is obtained in which the diamonds have
become concentrated. This passes at last into a remarkable machine
called the Greaser. The mixture of pebbles, which we may call
the concentrate, contains many minerals other than diamonds, such as
garnet, ilmenite, enstatite, chromite, zircon, kyanite, diopside and
half-a-dozen other species, varying in density from 2'6 to 5'3. When
this mixture flows in a current of water on to a series of sloping
cast-iron rocking plates covered with a thick layer of grease, the
diamonds adhere to the grease, while the other minerals, both those
which are heavier and those which are lighter than diamond, are carried
forward and away. Bits of metal and of iron pyrites do get embedded in
the grease along with diamonds, and if any corundum were present it
would also remain, but the separation of these substances from the
grease and from the diamonds is quite easy. The grease loses its
adhesive power by becoming superficially incorporated with minute
portions of water, and then needs remelting and re-spreading on the
oscillating " greasers." This discriminating process is based upon the
differing surface-attractions of certain minerals for water on the one
hand, and for oily and greasy materials on the other. In simpler words
diamonds and a few other minerals such as sapphires are apparently more
easily oiled than wetted, while the far greater number of minerals are
more easily wetted than oiled.
The
following table gives some particulars concerning a few of the
best-known and most important cut diamonds above 100
