August 5, 1897] 



NATURE 



329 



Laboratory diamonds burn in the air before the blowpipe to 

 carbonic acid ; and in lustre, crystalline form, optical proper- 

 ties, density, and hardness they are identical with the natural 

 stone. 



Many circumstances point to the conclusion that the diamond 

 of the chemist and the diamond of the mine are strangely akin 

 as to origin. It is conclusively proved that the diamond has 

 not been formed in situ in the blue ground. The diamond 

 genesis must have taken place at great depths under enormous 

 pressure. The explosion of large diamonds on coming to the 

 surface shows extreme tension. More diamonds are found in 

 fragments and splinters than in perfect crystals ; and it is note- 

 worthy that although many of these splinters and fragments are 

 derived from the breaking up of a large crystal, yet in no instance 

 have pieces been found which could be fitted together. Does 

 not this fact point to the conclusion that the blue ground is not 

 their true matrix ? Nature does not make fragments of crj'stals. 

 As the edges of the crystals are still sharp and unabraded the 

 ioitis of formation cannot have been very distant from the 



Fig. 5. — Diamond artificially crystallised from molten iron 



present sites. There were probably many sites of crystallisa- 

 tion differing in place and time, or we should not see such dis- 

 tinctive characters in the gems from different mines, nor indeed 

 in the diamonds from different parts of the same mine. 



The Mechanism of the Diamantiferous Pipes. 



How the great diamond pipes originally came into existence 

 is not difficult to understand in the light of the foregoing facts. 

 They certainly were not burst through in the ordinary manner 

 of volcanic eruption ; the surrounding and enclosing walls show- 

 no signs of igneous action, and are not shattered nor broken 

 even when touching the "blue ground." These pipes after 

 they were pierced were filled from below, and the diamonds 

 formed at some previous epoch too remote to imagine were 

 erupted with a mud volcano, together with all kinds of debris 

 eroded from the adjacent rocks. The direction of flow is seen 

 in the upturned edges of some of the strata of shale in the walls, 

 although I was unable at great depths to see any upturning in 

 most parts of the walls of the De Beers mine. 



Let me again refer you to the section through the Kimberley 

 mine. There are many such pipes in the immediate neighbour- 

 hood. It may be that each volcanic pipe is the vent for its own 

 special laboratory— a laboratory buried at vastly greater depths 

 than we have reached or are likely to reach — where the tem- 

 perature is comparable with that of the electric furnace, where 

 the pressure is fiercer than in our puny laboratories and the 

 melting-point higher, where no oxygen is present, and where 

 masses of carbon -saturated iron have taken centuries, perhaps 

 thousands of years, to cool to the solidif}'ing point. Such being 

 the conditions, the wonder is, not that diamonds are found as 

 big as one's fist, but that they are not found as big as one's head. 

 The chemist arduously manufactures infinitesimal diamonds, 

 valueless as ornamental gems ; but nature, with unlimited tem- 

 perature, inconceivable pressure, and gigantic material, to say 

 nothing of measureless time, produces without stint the dazzling, 

 radiant, beautiful crystals I am enabled to show you to-night, 



The ferric origin of the diamond is corroborated in many 

 ways. The country round Kimberley is remarkable for its 

 ferruginous character, and iron-saturated soil is popularly 

 regarded as one of the indications of the near presence of 

 diamonds. Certain artificial diamonds present the appearance 



NO. 1449, VOL. 56] 



of an elongated drop. From Kimberley I have with me 

 diamonds which have exactly the appearance of drops of liquid 

 separated in a pasty condition and crystallised on cooling. At 

 Kimberley, and in other parts of the world, diamonds have been 

 found with little appearance of crystallisation, but with rounded 

 forms similar to those which a liquid might assume if kept in 

 the midst of another liquid with which it would not mix. 

 Other drops of liquid carbon retained above their melting-point 

 for sufficient time would coalesce with adjacent drops, and on 

 slow cooling would separate in the form of large perfect crystaH>^ 

 Two drops, joining after incipient crystallisation, would assui^ 

 the not uncommon form of interpenetrating twin crystals: 

 Illustrations of these forms from Kimberley are here to-night. 

 Other modified circumstances would produce diamonds pre- 

 senting a confused mass of boarty crystals, rounded and 

 amorphous masses, or a hard black form of carbonado. 



Again, diamond crystals are almost invariably perfect on all 

 sides. They show no irreguhr side or face by which they were 

 attached to a support, as do artificial crystals of chemical salts ; 

 another proof that the diamond must have crystallised from a 

 dense liquid. 



When raised the diamond is in a state of enormous strain, 

 as I have already shown by means of polarised light. Some 

 diamonds exhibit cavities which the same test proves to contain 

 gas at considerable pressure. 



The ash left after burning a diamond invariably contains iron 

 as its chief constituent ; and the most common colours of dia- 

 monds, when not perfectly pellucid, show various shades of 

 brown and yellow, from the palest "off colour" to almost 

 black. These variations accord with the theory that the dia- 

 mond has separated from molten iron, and also explains how it 

 happens that stones from different mines, and even from dif- 

 ferent parts of the same mine, differ from each other. Along 

 with carbon, molten iron dissolves other bodies which possess 

 tinctorial powers. One batch of iron might contain an impurity 

 colouring the stones blue, another lot would tend towards the 

 formation of pink stones, another of green, and so on. 

 Traces of cobalt, nickel, chromium, and manganese — all 

 metals present in the blue ground — might produce all these 

 colours. 



An hypothesis, however, is of little value if it only elucidates 

 one half of a problem. Let us see how far we can follow out 

 the ferric hypothesis to explain the volcanic pipes. In the first 

 place we must remember these so-called volcanic vents are 

 admittedly not filled with the eruptive rocks, scoriaceous frag- 

 ments, &c. , constituting the ordinary contents of volcanic ducts. 

 At Kimberley the pipes are filled with a geological plum-pud- 

 ding of heterogeneous character — agreeing, however, in one 

 particular. The appearance of shale and fragments of other 

 rocks shows that the melange has suffered no great heat in its 

 present condition, and that it has been erupted from great 

 depths by the agency of water vapour or some similar gas. 

 How is this to be accounted for ? 



It must be borne in mind I start with the reasonable supposi- 

 tion that at a sufficient depth^ there were masses of molten iron 

 at great pressure and high temperature, holding carbon in 

 solution, ready to crystallise out on cooling. In illustration I may 

 cite the masses of erupted iron in Greenland. Far back in time the 

 cooling from above caused cracks in superjacent strata through 

 which water - found its way. Before reaching the iron the water 

 would be converted into gas, and this gas would rapidly dis- 

 integrate and erode the channels through which it passed, 

 grooving a passage more and more vertical in the endeavour 

 to find the quickest vent to the surface. But steam in the 

 presence of molten or even red-hot iron rapidly attacks it, 

 oxidises the metal and liberates large volumes of hydrogen gas, 

 together with less quantities of hydrocarbons * of all kinds- 

 liquid, gaseous, and solid. Erosion commenced by steam 

 would be continued by the other gases, and it would be no 

 difficult task for pipes, large as any found in South Africa, to 

 be scored out in this manner. Sir Andrew Noble has shown 

 that when the screw-stopper of his steel cylinders in which 

 gunpowder explodes under pressure is not absolutely perfect, 



1 The requisite pressure of fifteen tons on the square inch would exist not 

 many miles beneath the surface of the earth. 



■•' There are abundant signs that a considerable portion of this part of 

 Africa was once under water, and a fresh-water shell has been found in 

 apparently undisturbed blue ground at Kimberley. 



^ The water sunk in wells close to the Kimberley mine is sometimes 

 impregnated with paraffin, and Sir H. Roscoe extracted a solid hydro- 

 carbon from the "blue ground." 



