Gem-Stones and Their Distinctive Characters

by George Frederick Herbert Smith

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·O and 3·O. 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.

Figs. 57—59.—Diamond Crystals.

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 (Röntgen) 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 likewise 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 ἀδάμας, 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.