PRECIOUS STONES 91
color,
was known as " yellow earth," or " yellow ground," as heretofore
described. The crystals are distributed very irregularly, though the
form of them varies so distinctly with the different claims, that men
acquainted with the mines can generally tell, by the appearance of the
crystal, from which mine it was taken.
The
opinion of Sir William Crookes as to the origin of the mines appears,
from all the known facts, to be reasonable. He assumes that
deep-seated masses of molten iron, holding carbon in solution, were
confined at a very high temperature under enormous pressure. By these
forces, and a process of cooling continued during ages, the carbon was
crystallized and finally, by volcanic energy, forced through
intervening strata to the surface of the earth.
In
the early days these mines were worked as open quarried. Now shafts
are sunk near the pipes, tunnels and galleries are driven in the
diamond-bearing clay, and the material is run in trucks to the shaft
and hoisted to the surface. It is then lifted to a high platform and
allowed to fall to the ground. By this means the earth is broken until
it is the size of nut coal when it is searched for the larger diamonds.
Later, the stuff is passed over a separator, which is a machine of six
plates. These plates are covered with fat, to which the diamonds
adhere. Mr. Edwin W. Streeter says of this, " We have, on the authority
of Mr. E. D. Rudd, who has just returned from South Africa, the
remarkable statement that ninety per cent, of the diamonds contained in
the blue earth are found on the first plate, and he has never known of
one being found below the second plate."
The yield averages about three-fourths carat to the load of sixteen hundred pounds of clay.
The
relative value of the rough from the various mines, according to the
prices paid for it during a period of years, and taking the price of
Rivers at 100 as a standard, is as follows:
