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PRECIOUS STONES.

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