that high
temperature and pressure and the absence of oxidizing agents are
necessary conditions. The presence of sulphur, nickel, &c., in the
iron appears to favour the production of the diamond. On the other
hand, the occurrence in meteoric stones, and the experiments mentioned
above, show that the diamond may also crystallize from a basic magma,
capable of yielding some of the metallic oxides and ferro-magnesian
silicates; a magma, therefore, which is not devoid of oxygen. This is
still more forcibly suggested by the remarkable eclogite boulder found
in the blue ground of the Newlands mine, not far from the Vaal river,
and described by T. G. Bonney. The boulder is a crystalline rock
consisting of pyroxene (chrome-diopside), garnet, and a little
olivine, and is studded with diamond crystals; a portion of it is
preserved in the British Museum (Natural History). In another eclogite
boulder, diamond was found partly embedded in pyrope. Similar boulders
have also been found in the blue ground elsewhere. Specimens of pyrope
with attached or embedded diamond had previously been found in the
blue ground of the De Beers mines. In the Newlands boulder the
diamonds have the appearance of being an original constituent of the
eclogite. It seems therefore that a holocrystalline pyroxene-garnet
rock may be one source of the diamond found in blue ground. On the
other hand many tons of the somewhat similar eclogite in the De Beers
mine have been crushed and have not yielded diamond. Further, the
ilmenite, which is the most characteristic associate of the diamond in
blue ground, and other of the accompanying minerals, may have come
from basic rocks of a different nature.

The Inverell occurrence may prove to be another example of diamond
crystallized from a basic rock.

In both occurrences, however, there is still the possibility that the
eclogite or the basalt is not the original matrix, but may have caught
up the already formed diamond from some other matrix. Some regard the
eclogite boulders as derived from deep-seated crystalline rocks,
others as concretions in the blue ground.

None of the inclusions in the diamond gives any clue to its origin;
diamond itself has been found as an inclusion, as have also black
specks of some carbonaceous materials. Other black specks have been
identified as haematite and ilmenite; gold has also been found; other
included m