It is singular that in the early days of the dry diggings the diggers used in joke to express the very same opinions as I have quoted above, for they often unearthed diamonds coated with a thin, dark mineral crust, which disguised merely the shape of the diamond, not the color and lustre. Many diamonds were lost through this. After some months, exposure, or the attrition caused in again disturbing the soil in which they had been lying, the whole or part of the coating was so rubbed off that the diamond and its lustre became exposed to the digger’s delighted gaze. Most probably the idea of the ancients, as to the growth of diamonds, originated in the same or a closely similar manner.
Dana says: “The origin of the diamond has been a subject of speculation, and it is the prevalent opinion that the carbon, like that of coal, is of vegetable origin. Some crystals have been found with black uncrystallized particles or seams within, looking like coal, and this fact has been supposed to prove their vegetable origin.”
I take the following “New facts concerning the Diamond” from the Quarterly Journal of Science, Oct. 1873:
“Whilst our knowledge of the modes of formation of other gems is so rapidly advancing that the time does not seem to be very distant when the chemist in his laboratory will be able to produce them artificially if not in large at all events in microscopic crystals—the origin and mode of formation of the diamond is shrouded in apparently inexplicable mystery. It is even undecided whether the diamond is of igneous or vegetable origin, whether its nature is mineral or organic. Some diamonds appear to have been soft, as they are superficially impressed by sand and crystals; others contain crystals of other minerals, germs of plants, and fragments of vegetation. Professor Goppert has a diamond containing dendrites, such as occurs on minerals of aqueous origin, and there is at Berlin a diamond which contains bodies resembling protococcus pluvialis, and another containing green corpuscles linked together, closely resembling polinogtœa macrococca. (Palmoglœa Micrographic Dictionary.) Sir John Herschel quotes the case of a Bahia diamond mentioned by Harting, which contained well-formed filaments of iron pyrites. Messrs. Sorby and Baker have shown that the diamond may contain cavities entirely or partially filled with a liquid, probably condensed carbonic acid, and that the black specks in diamonds are really crystals which are sometimes surrounded by contraction cracks, a black cross appearing under polarised light. Sir David Brewster has likewise pointed out that the diamond possesses strata of different reflective powers. M. Damour states that diamonds sometimes contain spangles of gold in their cavities.... When shielded from contact with the air, the diamond may be exposed to the highest temperature of our furnaces without undergoing alteration, at least in the case of the colorless diamond; of colored diamonds more will be said hereafter.... A crystal of diamond, inclosed in a piece of dense coke and placed in a plumbago crucible packed with charcoal powder, was heated for half an hour in one of Siemens’ regenerative furnaces to the temperature at which cast-iron melts, without undergoing any change whatever. Another diamond, a cut (rose) diamond, which was inclosed in a crucible as before and heated for ten minutes in the furnace to a temperature at which wrought-iron melts, retained its form and the smoothness of its facets but became quite black and opaque, and exhibited a strong metallic lustre. The black portion formed a distinct layer of the thickness of a hair covering the unaltered substance within. These results confirm those of Schrötter, and appear to justify the view that diamond, though it undergoes no change when exposed to the greatest heat of a porcelain furnace or that at which cast-iron melts, is slowly converted at the temperature of molten wrought-iron into graphite. G. Rose states that some of the specimens of diamond in the Berlin collection appear quite black by reflected, though translucent by transmitted light, and that this black substance lying in the little irregularities of the surface is found by its behavior in fused nitre to be graphite. The relative ease with which graphite and diamond burn was determined by exposing them to the same temperature for the same time, when the following amounts of the three specimens were consumed:
| Foliated graphite | 27.45 | per | cent. |
| Diamond | 97.76 | „ | „ |
| Granular massive graphite | 100.00 | „ | „ |
“In a superb cut diamond weighing between six and seven carats, the brilliancy of the stone was decidedly increased after the operation. The loss of brilliancy observed by Mr. Schrötter is a proof, in M. Baumhauer’s opinion, that notwithstanding the precautions employed, the diamond had come in contact with the oxygen of the air, or else that at so elevated a temperature a reducing action had been effected upon the magnesia (in which the diamond had been packed) by the diamond, which had then been superficially burnt by the oxygen of that earth.
A diamond which presented to the naked eye an appearance of dirty green was treated in a similar manner; examination with a lens showed that the color did not extend to the entire stone, but was confined to small portions, which formed small green clouds in the centre of the mass. After heating to a white heat in hydrogen, the brilliancy of the surface remained as before; the transparency was rather increased than diminished, but the green hue was transformed into pale yellow. Another small diamond, of so dark a green as to approach black, and almost opaque, assumed a violet hue, retaining, however, its brilliancy, and becoming more translucid. A small cubic diamond of light green color preserved its brilliancy and transparency intact, but lost its color completely. No difference in its weight before and after the operation could be perceived.
“Brown diamonds lose most of their color when heated to whiteness in hydrogen; they generally assume a grayish tint, and in all cases the shade is much lighter, and on examination with a lens they appear limpid, with black spots. Diamonds with a yellow tint, such as Cape diamonds almost invariably are, scarcely lose any portion of their natural color.... Several experiments were made by von Baumhauer, in concert with M. Daniels, upon gray diamonds, in the hope that the effect of heat would, by removing their color, add to their value; but, unfortunately, the desired result was not achieved, as the diamonds presented after treatment the same grayish appearance as before. Very different results are obtained when, instead of heating the diamond in an atmosphere of hydrogen, it is heated in contact with the air. It is unnecessary to employ a white heat, or to subject the diamond to it for so long a time, in order to render it dull, and consequently opaque; this being the result of positive combustion, which is proved by its loss of weight after the operation.