Gem-Stones and Their Distinctive Characters

by George Frederick Herbert Smith

DIAMOND-SORTING MACHINES

PLATE XXIV

KAFIRS PICKING OUT DIAMONDS

The classification at the mines is first into groups by the shape: (1) close goods, (2) spotted stones, (3) rejection cleavage, (4) fine cleavage, (5) light brown cleavage, (6) ordinary and rejection cleavage, (7) flats, (8) macles, (9) rubbish, (10) boart. Close goods are whole crystals which contain no flaws and can be cut into single stones. Spotted stones, as their name suggests, contain spots which necessitate removal, and cleavage includes stones which are so full of flaws that they have to be cleaved or split into two or more stones. Flats are distorted octahedra, and macles are twinned octahedra. Rubbish is material which can be utilized only for grinding purposes, and boart consists of round dark stones which are invaluable for rock-drills. These groups are afterwards graded into the following subdivisions, depending on increasing depth of yellowish tint: (a) blue-white, (b) first Cape, (c) second Cape, (d) first bye, (e) second bye, (f) off-colour, (g) light yellow, (h) yellow. It is, however, only the first group that is so minutely subdivided. After being purchased, the parcels are split up again somewhat differently for the London market (cf. [p. 136]), and the dealers re-arrange the stones according to the purpose for which they are required. Formerly a syndicate of London merchants took the whole of the produce of the Kimberley mines at a previously arranged price per carat, but at the present time the diamonds are sold by certain London firms on commission.

The products of each mine show differences in either form or colour which enable an expert readily to recognize their origin. The old diggings by the Vaal River yielded finer and more colourless stones than those found in the dry diggings and the mines underlying them. The South African diamonds, taken as a whole, are always slightly yellowish or ‘off-coloured’; the mines are, indeed, remarkable for the number of fine and large, canary-yellow and brown, stones produced. The Kimberley mine yields a fair percentage of white, and a large number of twinned and yellow stones. The yield of the De Beers mine comprises mostly tinted stones—yellow and brown, occasionally silver capes, and very seldom stones free from colour. The Dutoitspan mine is noted for its harvest of large yellow diamonds; it also produces fine white cleavage and small white octahedra. The stones found in the Bultfontein mine are small and spotted, but, on the other hand, the yield has been unusually regular. The Premier or Wesselton mine yields a large proportion of flawless octahedra, but, above all, a large number of beautiful deep-orange diamonds. Of all the South African mines the Jagersfontein in the Orange River Colony alone supplies stones of the highly-prized blue-white colour and steely lustre characteristic of the old Indian stones. The new Premier mine in the Transvaal is prolific, but mostly in off-coloured and low-grade stones, the Cullinan diamond being a remarkable exception.

To illustrate the amazing productiveness of the South African mines, it may be mentioned that, according to Gardner F. Williams, the Kimberley group of mines in sixteen years yielded 36 million carats of diamonds, and the annual output of the Jagersfontein mine averages about a quarter of a million carats, whereas the total output of the Brazil mines, for the whole of the long period during which they have been worked, barely exceeds 13 million carats. The average yield of the South African mines, however, perceptibly diminishes as the depth of the mines increases.

The most interesting point connected with the South African diamond mines, viewed from the scientific standpoint, is the light that they have thrown on the question of the origin of the diamond, which previously was an incomprehensible and apparently insoluble problem. In the older mines, just as at the river diggings by the Vaal, the stones are found in a gravelly deposit that has resulted from the disintegration of the rocks through which the adjacent river has passed, and it is clear that the diamond cannot have been formed in situ here; it had been suspected, and now there is no doubt, that the itacolumite rock of Brazil has consolidated round the diamonds which are scattered through it, and that it cannot be the parent rock. The occurrence at Kimberley is very different. These mines are funnels which go downwards to unknown depths; they are more or less oval in section, becoming narrower with increasing depth, and are evidently the result of some eruptive agency. The Kimberley mine has been worked to a depth of nearly 4000 feet (1200 m.), and no signs of a termination have as yet appeared. The blue ground which fills these ‘pipes,’ as they are termed, must have been forced up from below, since it is sharply differentiated from the surrounding country rocks. This blue ground is a brecciated peridotite of peculiar constitution, to which the well-known petrologist, Carvil Lewis, who made a careful study of it, gave the name kimberlite. The blue colour testifies to its richness in iron, and it is to the oxidation of the iron constituent, that the change of colour to yellow in the upper levels is due. Owing to the shafts that have been sunk for working the mines, the nature of the surrounding rocks is known to some depth. Immediately below the surface is a decomposed ferriferous basalt, about 20 to 90 feet (6–27 m.) thick, next a black slaty shale, 200 to 250 feet (60–75 m.) thick, then 10 feet (3 m.) of conglomerate, next 400 feet (120 m.) of olivine diabase, then quartzite, about 400 feet (120 m.) thick, and lastly a quartz porphyry, which has not yet been penetrated. The strata run nearly horizontal, and there are no signs of upward bending at the pipes. The whole of the country, including the mines, was covered with a red sandy soil, and there was nothing to indicate the wealth that lay underneath. The action of water had in process of time removed all signs of eruptive activity. The principal minerals which are associated with diamond in the blue ground are magnetite, ilmenite, chromic pyrope, which is put on the market as a gem under the misnomer ‘Cape-ruby,’ ferriferous enstatite, which also is sometimes cut, olivine more or less decomposed, zircon, kyanite, and mica.

The evidence produced by an examination of the blue ground and the walls of the pipes proves that the pipes cannot have been volcanoes such as Vesuvius. There is no indication whatever of the action of any excessive temperature, while, on the other hand, there is every sign of the operation of enormous pressure; the diamonds often contain liquid drops of carbonic acid. Crookes puts forward the plausible theory that steam has been the primary agency in propelling the diamond and its associates up into the channel through which it has carved its way to freedom, and holds that molten iron has been the solvent for carbon which has crystallized out as diamond under the enormous pressures obtaining in remote depths of the earth’s crust. It is pertinent to note that, by dissolving carbon in molten iron, the eminent chemist, Moissan, was enabled to manufacture tiny diamond crystals. Water trickling down from above would be immediately converted into steam at very high pressure on coming into contact with the molten iron, and, in its efforts to escape, the steam would drive the iron and its precious contents, together with the adjacent rocks, upwards to the surface. The ferriferous nature of the blue ground and the yellow tinge so common to the diamonds lend confirmation to this theory. The process by which the carbon was extracted from shales or other carboniferous rocks and dissolved in iron still awaits elucidation.

Diamonds were found in New South Wales as long ago as 1851 on Turon River and at Reedy Creek, near Bathurst, about ninety miles (145 km.) from Sydney, but the find was of little commercial importance. A more extensive deposit came to light in 1867 farther north at Mudgee. In 1872 diamonds were discovered in the extreme north of the State, at Bingara near the Queensland border. Another discovery was made in 1884 at Tingha, and still more recently in the tin gravels of Inverell in the same region. In their freedom from colour and absence of twinning the New South Wales diamonds resemble the Brazilian stones. The average size is small, running about five to the carat when cut; the largest found weighed nearly 6 carats when cut. They are remarkable for their excessive hardness; they can be cut only with their own dust, ordinary diamond dust making no impression.