Pipes similar to those which surround Kimberley have been found in
other parts of S. Africa. One of the best known is that of
Jagersfontein, which was really the first of the dry diggings
(discovered in 1870). This large mine is near Fauresmith and 80 m. to
the south of Kimberley. In 1905 the year's production from the Orange
River Colony mines was more than 320,000 carats, valued at L938,000.
But by far the largest of all the pipes hitherto discovered is the
Premier mine in the Transvaal, about 300 m. to the east of Kimberley.
This was discovered in 1902 and occupies an area of about 75 acres. In
1906 it was being worked as a shallow open mine; but the description
of the Kimberley methods given above is applicable to the washing
plant at that time being introduced into the Premier mine upon a very
large scale. Comparatively few of the pipes which have been discovered
are at all rich in diamonds, and many are quite barren; some are
filled with "hard blue" which even if diamantiferous may be too
expensive to work.
The most competent S. African geologists believe all these remarkable
pipes to be connected with volcanic outbursts which occurred over the
whole of S. Africa during the Cretaceous period (after the deposition
of the Stormberg beds), and drilled these enormous craters through all
the later formations. With the true pipes are associated dykes and
fissures also filled with diamantiferous blue ground. It is only in
the more northerly part of the country that the pipes are filled with
blue ground (or "kimberlite"), and that they are diamantiferous; but
over a great part of Cape Colony have been discovered what are
probably similar pipes filled with agglomerates, breccias and tuffs,
and some with basic lavas; one, in particular, in the Riversdale
Division near the southern coast, being occupied by a melilite-basalt.
It is quite clear that the occurrence of the diamond in the S. African
pipes is quite different from the occurrences in alluvial deposits
which have been described above. The question of the origin of the
diamond in S. Africa and elsewhere is discussed below.
The River Diggings on the Vaal river are still worked upon a small
scale, but the production from this source is so limited that they are
of little account in comparison with the mines in the blue ground. The
stones, however, are good; since they differ somewhat from the
Kimberley crystals it is probable that they were not derived from the
present pipes. Another S. African locality must be mentioned;
considerable finds were reported in 1905 and 1906 from gravels at
Somabula near Gwelo in Rhodesia where the diamond is associated with
chrysoberyl, corundum (both sapphire and ruby), topaz, garnet,
ilmenite, staurolite, rutile, with pebbles of quartz, granite,
chlorite-schist, &c. Diamond has also been reported from kimberlite
"pipes" in Rhodesia.
_Other Localities._--In addition to the South American localities
mentioned above, small diamonds have also been mined since their
discovery in 1890 on the river Mazaruni in British Guiana, and finds
have been reported in the gold washings of Dutch Guiana. Borneo has
possessed a diamond industry since the island was first settled by the
Malays; the references in the works of Garcia de Orta, Linschoten, De
Boot, De Laet and others, to Malacca as a locality relate to Borneo.
The large Borneo stone, over 360 carats in weight, known as the Matan,
is in all probability not a diamond. The chief mines are situated on
the river Kapuas in the west and near Bandjarmassin in the south-east
of the island, and the alluvial deposits in which they occur are
worked by a small number of Chinese and Malays. Australia has yielded
diamonds in alluvial deposits near Bathurst (where the first discovery
was made in 1851) and Mudgee in New South Wales, and also near Bingara
and Inverell in the north of the colony. At Mount Werong a stone
weighing 29 carats was found in 1905. At Ruby Hill near Bingara they
were found in a breccia filling a volcanic pipe. At Ballina, in New
England, diamonds have been found in the sea sand. Other Australian
localities are Echunga in South Australia; Beechworth, Arena and
Melbourne in Victoria; Freemantle and Nullagine in Western Australia;
the Palmer and Gilbert rivers in Queensland. These have been for the
most part discoveries in alluvial deposits of the goldfields, and the
stones were small. In Tasmania also diamonds have been found in the
Corinna goldfields. Europe has produced few diamonds. Humboldt
searched for them in the Urals on account of the similarity of the
gold and platinum deposits to those of Brazil, and small diamonds were
ultimately found (1829) in the gold washings of Bissersk, and later at
Ekaterinburg and other spots in the Urals. In Lapland they have been
found in the sands of the Pasevig river. Siberia has yielded isolated
diamonds from the gold washings of Yenisei. In North America a few
small stones have been found in alluvial deposits, mostly auriferous,
in Georgia, N. and S. Carolina, Kentucky, Virginia, Tennessee,
Wisconsin, California, Oregon and Indiana. A crystal weighing 23-3/4
carats was found in Virginia in 1855, and one of 21-1/4 carats in
Wisconsin in 1886. In 1906 a number of small diamonds were discovered
in an altered peridotite somewhat resembling the S. African blue
ground, at Murfreesboro, Pike county, Arkansas. Considerable interest
attaches to the diamonds found in Wisconsin, Michigan and Ohio near
the Great Lakes, for they are here found in the terminal moraines of
the great glacial sheet which is supposed to have spread southwards
from the region of Hudson Bay; several of the drift minerals of the
diamantiferous region of Indiana have been identified as probably of
Canadian origin; no diamonds have however yet been found in the
intervening country of Ontario. A rock similar to the blue ground of
Kimberley has been found in the states of Kentucky and New York. The
occurrence of diamond in meteorites is described below.
_Origin of the Diamond in Nature._--It appears from the foregoing
account that at most localities the diamond is found in alluvial
deposits probably far from the place where it originated. The minerals
associated with it do not afford much clue to the original conditions;
they are mostly heavy minerals derived from the neighbouring rocks, in
which the diamond itself has not been observed. Among the commonest
associates of the diamond are quartz, topaz, tourmaline, rutile,
zircon, magnetite, garnet, spinel and other minerals which are common
accessory constituents of granite, gneiss and the crystalline schists.
Gold (also platinum) is a not infrequent associate, but this may only
mean that the sands in which the diamond is found have been searched
because they were known to be auriferous; also that both gold and
diamond are among the most durable of minerals and may have survived
from ancient rocks of which other traces have been lost.
The localities at which the diamond has been supposed to occur in its
original matrix are the following:--at Wajra Karur, in the Cuddapah
district, India, M. Chaper found diamond with corundum in a decomposed
red pegmatite vein in gneiss. At S[=a]o Joao da Chapada, in Minas
Geraes, diamonds occur in a clay interstratified with the itacolumite,
and are accompanied by sharp crystals of rutile and haematite in the
neighbourhood of decomposed quartz veins which intersect the
itacolumite. It has been suggested that these three minerals were
originally formed in the quartz veins. In both these occurrences the
evidence is certainly not sufficient to establish the presence of an
original matrix. At Inverell in New South Wales a diamond (1906) has
been found embedded in a hornblende diabase which is described as a
dyke intersecting the granite. Finally there is the remarkable
occurrence in the blue ground of the African pipes.
There has been much controversy concerning the nature and origin of
the blue ground itself; and even granted that (as is generally
believed) the blue ground is a much serpentinized volcanic breccia
consisting originally of an olivine-bronzite-biotite rock (the
so-called kimberlite), it contains so many rounded and angular
fragments of various rocks and minerals that it is difficult to say
which of them may have belonged to the original rock, and whether any
were formed _in situ_, or were brought up from below as inclusions.
Carvill Lewis believed the blue ground to be true eruptive rock, and
the carbon to have been derived from the bituminous shales of which it
contains fragments. The Kimberley shales, which are penetrated by the
De Beers group of pipes, were, however, certainly not the source of
the carbon at the Premier (Transvaal) mine, for at this locality the
shales do not exist. The view that the diamond may have crystallized
out from solution in its present matrix receives some support from the
experiments of W. Luzi, who found that it can be corroded by the
solvent action of fused blue ground; from the experiments of J.
Friedlander, who obtained diamond by dissolving graphite in fused
olivine; and still more from the experiments of R. von Hasslinger and
J. Wolff, who have obtained it by dissolving graphite in a fused
mixture of silicates having approximately the composition of the blue
ground. E. Cohen, who regarded the pipes as of the nature of a mud
volcano, and the blue ground as a kimberlite breccia altered by
hydrothermal action, thought that the diamond and accompanying
minerals had been brought up from deep-seated crystalline schists.
Other authors have sought the origin of the diamond in the action of
the hydrated magnesian silicates on hydrocarbons derived from
bituminous schists, or in the decomposition of metallic carbides.
Of great scientific interest in this connexion is the discovery of
small diamonds in certain meteorites, both stones and irons; for
example, in the stone which fell at Novo-Urei in Penza, Russia, in
1886, in a stone found at Carcote in Chile, and in the iron found at
Canon Diablo in Arizona. Graphitic carbon in cubic form (cliftonite)
has also been found in certain meteoric "irons," for example in those
from Magura in Szepes county, Hungary, and Youndegin near York in
Western Australia. The latter is now generally believed to be altered
diamond. The fact that H. Moissan has produced the diamond
artificially, by allowing dissolved carbon to crystallize out at a
high temperature and pressure from molten iron, coupled with the
occurrence in meteoric iron, has led Sir William Crookes and others to
conclude that the mineral may have been derived from deep-seated iron
containing carbon in solution (see the article GEM, ARTIFICIAL). Adolf
Knop suggested that this may have first yielded hydrocarbons by
contact with water, and that from these the crystalline diamond has
been formed. The meteoric occurrence has even suggested the fanciful
notion that all diamonds were originally derived from meteorites. The
meteoric iron of Arizona, some of which contains diamond, is actually
found in and about a huge crater which is supposed by some to have
been formed by an immense meteorite penetrating the earth's crust.
It is, at any rate, established that carbon can crystallize as diamond
from solution in iron, and other metals; and it seems 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 minerals recorded are rutile, topaz, quartz, pyrites,
apophyllite, and green scales of chlorite (?). Some of these are of
very doubtful identification; others (e.g. apophyllite and chlorite)
may have been introduced along cracks. Some of the fibrous inclusions
were identified by H. R. Goppert as vegetable structures and were
supposed to point to an organic origin, but this view is no longer
held. Liquid inclusions, some of which are certainly carbon dioxide,
have also been observed.
Finally, then, both experiment and the natural occurrence in rocks and
meteorites suggest that diamond may crystallize not only from iron but
also from a basic silicate magma, possibly from various rocks
consisting of basic silicates. The blue ground of S. Africa may be
the result of the serpentinization of several such rocks, and
although now both brecciated and serpentinized some of these may have
been the original matrix. A circumstance often mentioned in support of
this view is the fact that the diamonds in one pipe generally differ
somewhat in character from those of another, even though they be near
neighbours.
_History._--All the famous diamonds of antiquity must have been Indian
stones. The first author who described the Indian mines at all fully was
the Portuguese, Garcia de Orta (1565), who was physician to the viceroy
of Goa. Before that time there were only legendary accounts like that of
Sindbad's "Valley of the Diamonds," or the tale of the stones found in
the brains of serpents. V. Ball thinks that the former legend originated
in the Indian practice of sacrificing cattle to the evil spirits when a
new mine is opened; birds of prey would naturally carry off the flesh,
and might give rise to the tale of the eagles carrying diamonds adhering
to the meat.