south African diamonds (Continued)
which
the stones have accidentally been washed by millenniums of rain. In
this blue ground are no less than eighty different minerals pushed up
together from within the earth by the heaving force of actions under
the crust. Presumably all diamonds once arose through such pipes in
India, Brazil and Africa. This is yet a theory and an unsolved problem.
The
"blue ground" of the South African pipes is a basic igneous rock; it
contains no quartz whatever. Igneous rocks, in general, are classified
according to the amount of quartz they contain. Quartz is one of the
commonest of minerals; each grain of sand on the seashore is made of
it. If an igneous rock contains lots of quartz, it is said to be
"acid"; if it contains little or none, it is "basic". This, it seems,
is one of the many conditions to be satisfied before a diamond can be
formed. The blue ground was composed, originally, of large amounts of
the glassy, grass-green mineral olivine, with smaller amounts of
scores of others; notably garnet, bronze-colored mica and dark, heavy
iron minerals. While this mass was cooling, the olivine was attacked
by chemical solutions, and most of it turned into serpentine—a dark
greenish-blue, rather soft mineral. Hence the color, and hence the
name, "blue ground".
The
chief use of most diamonds is for adornment. Today they come from one
continent, Africa, except in quantities so small as to be negligible.
Two-thirds of the diamonds in Africa are mined on British territory, a
few in Portuguese Angola, the rest in the Belgian Congo. Almost all the
diamonds taken out of Africa are sold by
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