Gems - Precious Stones - Diamonds
( Originally Published 1919 )
DIAMOND has held pride of place as chief of precious stones ever since the discovery of the form of cutting known as the ' brilliant' revealed to full perfection its amazing qualities ; and justly so, since it combines in itself extreme hardness, high refraction, large colour-dispersion, and brilliant lustre. A rough diamond, especially from river gravels, has often a peculiar greasy appearance, and is no more attractive to the eye than a piece of washing-soda. It is therefore easy to understand why the Persians in the thirteenth century placed the pearl, ruby, emerald, and even peridot before it,. and writers in the Middle Ages frequently esteemed it below emerald and ruby. The Indian lapidaries, who were the first to realize that diamond could be ground with its own powder, discovered what a wonderful difference the removal of the skin makes in the appearance of a stone. They, however, made no attempt to shape a stone, but merely polished the natural facets, and only added numerous small facets when they wished to conceal flaws or other imperfections ; indeed, the famous traveller, Tavernier, from whom most of our knowledge of early mining in India is obtained, invariably found that a stone covered with many facets was badly flawed. The full radiant beauty of a diamond comes to light only when it is cut in brilliant form.
Of all precious stones diamond has the simplest composition ; it is merely crystallized carbon, another form of which is the humble and useful graphite, commonly known as ` black-lead.' Surely nature has surpassed all her marvellous efforts in producing from the same element substances with such divergent characters as the hard, brilliant, and transparent diamond and the soft, dull, and opaque graphite. It is, however, impossible to draw any sharp dividing line between the two ; soft diamond passes insensibly into hard graphite, and vice versa. Boart, or bort, as it is sometimes written, is composed of minute crystals of diamond arranged haphazardly; it possesses no cleavage, its hardness is greater than that of the crystals, and its colour is greyish to blackish. Carbon, carbonado, or black diamond, which is composed of still more minute crystals, is black and opaque, and is perceptibly harder than the crystals. It passes into graphite, which varies in hardness, and may have any density between 2.0 and 3.0. Jewellers apply the term boart to crystals or fragments which are of no service as gems ; such pieces are crushed to powder and used for cutting and polishing purposes.
Diamonds, when absolutely limpid and free from flaws, are said to be of the ` first water,' and are most prized when devoid of any tinge of colour except perhaps bluish (Plate I, Fig. 1). Stones with a slight tinge of yellow. are termed ' off-coloured,' and are far less valuable. Those of a canary-yellow colour (Plate I, Fig. 3), however, belong to a different category, and have a decided attractiveness. Greenish stones also are common, though it is rare to come across one with a really good shade of that colour. Brown stones, especially in South Africa, are not uncommon. Pink stones are less common, and ruby-red and blue stones are rare. Those of the last-named colour have usually what is known as a ' steely' shade, i.e. they are tinged with green ; stones of a sapphire blue are very seldom met with, and such command high prices.
Diamond crystallizes (Figs. 57—59 and Plate I, Fig. 2) in octahedra with brilliant, smooth faces, and occasionally in cubes with rough pitted faces ; sometimes three or six faces take the place of each octahedron face, and the stone is almost spherical in shape. The surfaces of the crystals are often marked with equilateral triangles, which are supposed to represent the effects of incipient combustion. Twinned crystals, in which the two individuals may be connected by a single plane or may be interpenetrating, a star shape often resulting in the latter case, are common ; sometimes, if of the octahedron type, they are beautifully symmetrical. The rounded crystals are frequently covered with a peculiar gum-like skin which is somewhat less hard than the crystal itself. A large South African stone, weighing 27 grams (130 carats) and octahedral in shape, which was the gift of John Ruskin, and named by him the ` Colenso' after the first bishop of Natal, is exhibited in the British Museum (Natural History) ; its appearance is, however, marred by its distinctly `off-coloured' tint.
The refraction of diamond is single, but local double refraction is common, indicating a state of strain which can often be traced to an included drop of liquid carbonic acid ; so great is the strain that many a fine stone has burst to fragments on being removed from the ground in which it has lain. The refractive index for the yellow light of a sodium flame is 2'4175, and the slight variation from this mean value that has been observed, amounting only to 0.0001, testifies to the purity of the composition. The colour-dispersion is large, being as much as 0.044, in which respect it surpasses all colourless stones, but is exceeded by sphene and the green garnet from the Urals (cf. p. 217). The lustre of diamond, when polished, is so characteristic as to be termed adamantine, and is due to the combination of high refraction and extreme hardness. Diamond is translucent to the X (Rontgen) rays ; it phosphoresces under the action of radium, and of a high-tension electric current when placed in a vacuum tube, and sometimes even when exposed to strong sunlight. Some diamonds fluoresce in sunlight, turning milky, and a few even emit light when rubbed. Crookes found that a diamond buried in radium bromide for a year had acquired a lovely blue tint, which was not affected even by heating to redness. The specific gravity is like-wise constant, being 3.521, with a possible variation from that mean value of 0.005 ; but a greater range, as might be expected, is found in the impure boart.
Diamond is by far the hardest substance in nature, being marked 10 in Mohs's scale of hardness, but it varies in itself; stones from Borneo and New South Wales are so perceptibly harder than those usually in the lapidaries' hands, that they can be cut only with their own and not ordinary diamond powder, and some difficulty was experienced in cutting them when they first came into the market. It is interesting to note that the metal tantalum, the isolation of which in commercial amount constituted one of the triumphs of chemistry of recent years, has about the same hardness as diamond. Despite its extreme hardness diamond readily cleaves under a heavy blow in planes parallel to the faces of the regular octahedron, a property utilized for shaping the stone previous to cutting it. The fallacious, but not unnatural, idea was prevalent up to quite modern times. that a diamond would, even if placed on an anvil, resist a blow from a hammer : who knows how many fine stones have succumbed to this illusory test? The fact that diamond could be split was known to Indian lapidaries at the time of Tavernier's visit, and it would appear from De Boodt that in the sixteenth century the cleavability of diamond was not unknown in Europe, but it was not credited at the time and was soon forgotten. Early last century Wollaston, a famous chemist and mineralogist, rediscovered the property, and, so it is said, used his knowledge to some profit by purchasing large stones, which because of their awkward shape or the presence of flaws in the interior were rejected by the lapidaries, and selling them back again after cleaving them to suitable forms.
It has already been remarked (p. 79) that the interval in hardness between diamond and corundum, which comes next to it in Mohs's scale, is enormously greater than that between corundum and the softest of minerals. Diamond can therefore be cut only with the aid of its own powder, and the cutting of diamond is therefore differentiated from that of other stones, the precious-stone trade, being to a large extent divided into two distinct groups, namely, dealers in diamonds, and dealers in all other gem-stones.
The name of the species is derived from the popular form, adiamentem, of the Latin adamantem, itself the alliterative form of the Greek aSaµac, meaning the unconquerable, in allusion not merely to the great hardness but also to the mistaken idea already mentioned. Boart probably comes from the Old-French bord or bort, bastard.
At the present day diamonds are usually cut as brilliants, though the contour of the girdle may be circular, oval, or drop-shaped to suit the particular purpose for which the stone is required, or to keep the weight as great as possible. Small stones for bordering a large coloured stone may also be cut as roses or points. A perfect brilliant has 58 facets, but small stones may have not more than 44, and exceptionally large stones may with advantage have many more ; for instance, on the largest stone cut from the Cullinan diamond there are no fewer than 74 facets.
The description of the properties of diamond would not be complete without a reference to the other valuable, if utilitarian, purposes to which it is put. Without its aid much of modern engineering work and mining operations would be impossible except at the cost of almost prohibitive expenditure of time and money.
Boring through solid rock has been greatly facilitated by the use of the diamond drill. For this purpose carbonado or black diamond is more serviceable than single crystals, and the price of the former has consequently advanced from a nominal figure up to £3 to L12 a carat. The actual working part of the drill consists of a cast-steel ring. The crown of it has a number of small depressions at regular intervals into which the carbonados are embedded. On revolution of the drill an annular ring is cut, leaving a solid core which can be drawn to the surface. For cooling the drill and for washing away the detritus water is pumped through to the working face. The duration of the carbonados depends on the nature of the rock and the skill of the operator. The most troublesome rock is a sandstone or one with sharp differences in hardness, because the carbonados are liable to be torn out of their setting. An experienced operator can tell by the feel of the drill the nature of the rock at the working face, and by varying the pressure can mitigate, the risk of damage to the drill.
The tenacity of diamond renders it most suitable for wire-drawing. The tungsten filaments used in many of the latest forms of incandescent electric lamps are prepared in this manner.
Diamond powder is used for cutting and turning the hardened steel employed in modern armaments and for other more peaceful purposes.
Although nearly all the gem-stones scratch glass, diamond alone can be satisfactorily employed to cut it along a definite edge. Any flake at random will not be suitable, because it will tear the glass and form a jagged edge. The best results are given by the junction of two edges which do not meet in too obtuse an angle ; two edges of the rhombic dodecahedron meet the requirements admirably. The stones used by the glaziers are minute in size, being not much larger than a pin's head, and thirty of them on an average go to the carat. They are set in copper or brass. Some little skill is needed to obtain the best results.
The value of a diamond has always been determined largely by the size of the stone, the old rule being that the rate per carat should be multiplied by the square of the weight in carats ; thus, if the rate be £10, the cost of a two-carat stone is four times this sum, or £40, of a three-carat stone £90, and so on. For a century, from 1750 to 1850, the rate remained almost constant at £4 for rough, £6 for rose-cut, and £8 for brilliant-cut diamonds. Since the latter date, owing to the increase in the supply of gold, the growth of the spending power of the world, and the gradual falling off in the productiveness of the Brazilian fields, the rate steadily increased about 10 per cent. each year, until in 1865 the rate for brilliants was £18. The rise was checked by the discovery of the South African mines; moreover, since comparatively large stones are plentiful in these mines, the rule of the increase in the price of a stone by the square of its weight no longer holds. The rate for the most perfect stones still remains high, because such are not so common in the South African mines. The classification 1 adopted by the syndicate of London diamond merchants who place upon the market the output of the De Beers group of mines is as follows :—(a) Blue-white, (b) white, (c) silvery Cape, (d) fine Cape, (e) Cape, (f) fine by-water, (g) bywater, (h) fine light brown, (i) light brown, (j) brown, (k) dark brown. Bywaters or byes are stones tinged with yellow.
Melee are stones smaller than a quarter of a carat. It will be noticed that the prices depart largely from the old rule ; thus taking the rate for a carat blue-white stone, the price of a five-carat stone should be from £150—200 a carat, and for a quarter-carat stone only £7, 10s. to £10 a carat. There happens to be at the time of writing very little demand for five-carat stones. Of course, the prices given are subject to constant fluctuation depending upon the supply and demand, and the whims of fashion.