Diamonds And Luxury

An important character of precious stones not yet touched upon is colour. The beauty of opaque and lustreless stones, such as the turquoise, depends wholly upon this character. Every shade of colour is represented among minerals used as precious stones and for ornamental purposes. As has already been mentioned, stones which are perfectly colourless and also perfectly transparent are described as being water-clear, or of the first water.

In the majority of cases colour is a very variable character; there are, however, examples amongst minerals in which the colour is a fixed and essential character, appearing the same whether the mineral is in the largest masses or reduced to the finest powder. Such a stone would be described as being idiochromatic, it having a colour of its own, which is essential and characteristic of the mineral. As an example of an idiochromatic stone, malachite, which is always green whether in mass or in powder, may be quoted.
The majority of precious stones are, if perfectly pure, completely colourless. But this purity of composition is, as a rule, not attained, and thus, owing to the admixture of foreign colouring-matter, such stones occur much more frequently coloured than colourless. The colour, being thus due to accidental impurities, may vary in different specimens of the same stone, or even in different portions of the same specimen, and must therefore be regarded as a non-essential character. Stones in which colour is a variable character are distinguished as allochromatic, and the foreign matter, to which their colouring is due, may be regarded as pigment. Different pigments give rise to differently coloured specimens of the same mineral species. Such specimens show their colour best in fragments of some thickness ; in very thin splinters, or in fine powder, they appear only faintly coloured or even completely colourless.

The variety of colour exhibited by quartz well illustrates the fortuitous nature of this character when due to impurities. Thus, rock-crystal is transparent and water-clear quartz, smoky-quartz is brown, amethyst is violet quartz, citrine is yellow quartz, green quartz is known as plasma, blue quartz as sapphire-quartz, and there are still other coloured varieties, with special names. Again, the mineral corundum, which sometimes occurs colourless, is known as ruby when red and as sapphire when blue; it is also found of many other colours, which will be mentioned in the special description of corundum. Though diamond, in its most valuable condition, is water-clear, yet specimens of every shade of colour are found.

The range of colour shown by any one allochromatic mineral is known as its suite of colours. Thus the suite of colours shown by quartz includes brown, violet, yellow, green, blue, &c.; that by corundum includes red and blue and many others. The suite of colours shown by any one mineral will not usually be shown by any other; in nearly every case certain colours will be unrepresented in the suite.

In minerals in which the lustre is other than metallic, a group which comprises nearly all precious stones, eight principal colours may be recognised for the purposes of descriptive mineralogy; these colours are white, grey, black, blue, green, yellow, red, and brown. Intermediate colours may be described by terms compounded of the names of the eight principal colours ; as, for instance, red dish-white, greenish-blue, bluish-black, &c. 3 stone diamond ring for example. The different shades or tints shown by each of the principal colours are indicated by a descriptive prefix; as, for example, sulphur-yellow, grass-green, indigo-blue, smoke-grey, carmine-red, &c. The colour of a mineral can be judged more correctly by observing it close to the eye, when small differences in colour will be more apparent.
The character of the colour shown by a mineral depends partly on the lustre and transparency of the specimen; it may be described by terms in use in ordinary language, such as lively, warm, fresh, dull, delicate, soft, dirty, dusky, &c. The intensity of the colour shown by a mineral also varies in different specimens ; it may be described as deep or dark, when approaching to black ; high or full, when pure and intense ; light, when approaching to white; finally, as pale, when more nearly approaching to white. In speaking of some precious stones, for example the ruby, it was formerly the custom to describe specimens with a deep or full colour as " masculine," and those with a lighter colour as " feminine." These terms have now, however, fallen into disuse.
Intensity of colour depends on the amount of colouring-matter present; the greater this is, the deeper will be the colour of the stone. When the pigment of a stone is distributed equally throughout its mass, the stone will be uniformly coloured. If, on the contrary, the pigment is present in some parts and absent in others, or present in varying amounts in different parts of the stone, the latter will show corresponding differences in colour.

One and the same stone may be differently coloured in different parts owing to the presence of different pigments; thus sapphire often shows blue spots or patches on a colourless background, and amethyst may show violet areas also on a colourless background. The irregular distribution of colour in such stones detracts considerably from their beauty ; specimens of precious stones in which the colour is intense and distributed with perfect uniformity are therefore specially valuable.

The distribution of colour in any one kind of stone is sometimes remarkably constant, appearing repeatedly in a large number of specimens. Thus in the four-sided columns of diopside from the Zillerthal in the Tyrol, which are sometimes used as gems, one end is colourless and the other of a fine, dark, bottle-green colour. In the same way the hexagonal prisms of red, green, or almost colourless tourmaline from Elba frequently have a black termination (so-called negro-heads). A regular arrangement of different colours in the same crystals is sometimes seen in tourmaline, as illustrated in Plate XV., Figs. 8 and 9, where the central portion is red and the external portion green, the two colours being sharply separated from each other. The beauty of agate is due to the arrangement of its various colours in bands. The following terms are used in describing colour distribution : spotted, mottled, clouded, veined, marbled, striated, banded, &c.

Brown or black arborescent, or tree-like, markings are frequently seen in certain specimens of chalcedony, and are described as dendritic markings. Stones showing such markings are known as dendrites. They are cut and polished with the object of bringing out the markings as prominently as possible (Fig. 89). Dendrites, among which is moss-agate with its peculiar and moss-like distribution of green colouring-matter, will be further considered in dealing with opal, chalcedony, &c.

The various pigments to which the colouring of precious stones is due may be organic or inorganic, and differ much in character. They may exist in considerable amount, but more frequently they are present in such small quantities that very exact chemical analysis is necessary for their detection. In the latter case, the colouring-power of the pigment must be comparable to that of carmine and some other pigments, an extremely small quantity of which is capable of giving a decided colour to an enormous quantity of a colourless substance.

The precise nature of the colouring-matter of many precious stones it has been impossible as yet to determine. Large quantities of the precious stones would be needed to yield an amount of colouring-matter sufficient for a reliable analysis, and here lies the chief obstacle to the investigation. In spite of this difficulty it has been possible in some cases to determine definitely to what substance the colouration is due. gia precious stones. Thus, for example, the emerald owes its green colour to the presence, in small quantity, of a compound of the metal chromium, while the apple-green colour of chrysoprase is due to a compound of the metal nickel. Other stones are coloured by compounds of iron or copper ; while the brown colour of smoky-quartz is due to an organic substance, which can be distilled off as a dark brown oil, possessing an empyreumatic odour.

The colouring-matter of precious stones is frequently distributed so intimately and uniformly through their substance that it is impossible, with the strongest magnification, to distinguish single particles of the pigment. The relation between the substance of the precious stone and the pigment seems analogous to that which exists between a solvent and a substance dissolved in it. In such cases, it is inferred that the colouring-matter is not an essential constituent of the substance of the stone from the fact that specimens of other colours, or devoid of colour, are known. This intimate association of the pigment with the ground substance of the stone exists, for example, in the green emerald, in the blue opaque turquoise, the colour in the latter case being due to compounds of copper and iron ; as also in diopside, the green colour of which is given by a compound of ferrous oxide.

In most of these cases we are dealing with something more intimate than a mere mechanical mixture. The colouring-matter is isomorphous with the ground substance of the precious stone, that is, it has the same type of chemical formula, and when this is the case, the intermixing of the two substances involves not microscopically small particles of each but the ultimate particles or molecules of each substance. Thus to take diopside as an example, we must picture the molecules of the compound of ferrous oxide distributed uniformly between the molecules of the ground substance of the stone and imparting to it its characteristic green colour. The same may perhaps be said of emerald which belongs to the mineral species beryl, specimens of which sometimes occur colourless ; also of turquoise and many other precious stones.

In other stones, on the contrary, the colour is due to vast numbers of minute coloured particles, with definite boundaries, mechanically intermixed with the colourless ground-mass of the stone. These particles may be large enough to be just perceptible to the naked eye, or so small as to require a lens or microscope for their detection ; they may have the form of grains, scales, fibres, or needles. Small blue grains distributed in large numbers through the colourless ground-mass of lapis-lazuli give to this precious stone its fine blue colour. Green needles and fibres of the mineral actinolite give rise to the green colour of prase, a variety of quartz, which of itself is colourless. Felspar is sometimes coloured red by minute scales of iron-glance (hematite), and is then used as an ornamental stone under the name of sun-stone: chalcedony, coloured by a similar red pigment, is the much used carnelian.
Stones coloured in this manner, by the mechanical intermixture of particles of pigment, are more or less cloudy or even opaque; those, on the contrary, in which a more intimate or chemical relation between the colouring-matter and the ground-substance is possible, are clear and transparent.

The apparent change of colour shown by many precious stones when exposed to different kinds of illumination is worthy of remark. In most cases the colour seen in clear day-light is the most beautiful, the appearance by artificial light being less pleasing. Thus amethyst by day-light is of a beautiful purple colour, but in candle-light it appears dull grey. Purple corundum or " oriental amethyst,11 on the contrary, shows its fine colour as well by candle-light as by day-light. Specially peculiar in this respect is the variety of chrysoberyl, known as alexandrite, which, as we shall see later on, is green in day-light and red in candle-light. Yellow diamonds retain their colour in the electric light, but appear colourless in candle-light. Many other stones afford similar examples of a change in colour accompanied by a loss of beauty in artificial light, a property which naturally diminishes the value they might otherwise possess.

The possibility of temporarily masking the colour of yellowish diamonds has, in recent years, frequently led to fraud. Since the discovery of the South African mines, yellowish diamonds are fairly abundant, and therefore comparatively cheap, while perfectly colourless stones command a high price. By giving these yellowish stones a very thin coating of some blue colouring-matter, they can be made to appear colourless, the mixture of blue and yellow light rays producing on the eye the effect of white light. As soon, however, as the blue coating is worn off the fraud becomes apparent.

Not only an apparent, but an actual change of colour, may be experienced by some precious stones. As a rule, the colours of precious stones are extremely lasting, only dis¬appearing with the destruction of the stone itself, this being, for example, the case with the yellow diamond, the ruby, emerald, and others. The colour of other stones, however, is less constant, and may be completely destroyed, the substance of the stone undergoing no change in the process. The colouring of such a stone will frequently disappear when the stone is raised to a red heat or even less; this will invariably happen if the colouring-matter is organic in nature, since it will be decomposed at such a temperature. Brown smoky-quartz and reddish-yellow hyacinth behave in this way, becoming completely colourless when heated to redness. Other stones on being heated experience not a loss but a change of colour; thus the violet amethyst becomes yellow, and the dark yellow topaz becomes rose-red in colour. These particular changes in colour are sometimes brought about intentionally in order to obtain yellow quartz (" burnt amethyst") and rose-red topaz (rose topaz), both of which are used as cut stones, but occur in nature to only a small extent.

Many stones show characteristic changes in colour during the progress of a rise and fall in temperature. Thus the red ruby, at a high temperature, is colourless ; on cooling it first becomes green, after which it gradually assumes its original fine red colour. The red spinel behaves somewhat differently under similar conditions; at a high temperature it becomes colourless, and on cooling it regains its original colour, so far resembling the behaviour of the ruby, but at the intermediate temperature it assumes not a green but a violet tint. A high temperature is not invariably necessary to effect a change of colour in precious stones; some stones are so sensitive that their colour fades or disappears merely on exposure to light and air. Certain topazes behave in this way, and after a few months exposure will be recognisably paler in colour ; the same phenomenon may be observed in green chrysoprase and in rose-quartz, as well as in some blue turquoises, the colour of the latter of which may gradually change to green. Obviously the value of such stones will be considerably diminished, since it is difficult, if not impossible, to avoid a rapid loss of colour, and there¬fore of beauty, when they are used under ordinary conditions.

Colour lost in this way may sometimes be restored by keeping the stone in darkness, by burying it in moist earth, or by treating it with certain chemicals, all of which devices are made use of by unscrupulous dealers. As a contrast to the behaviour of such stones on exposure to light, it may be mentioned here that amber, instead of being bleached by exposure, is darkened, gradually becoming of a dark, reddish-brown colour.

The artificial colouring: or recolouring of precious stones, which was known and practised to some extent among the ancients, is of some importance. At the present day agate and similar stones are most frequently subjected to this treatment, the exact methods of which will be dealt with under the special description of these stones. The capacity for absorbing the liquid which imparts its colour to the stone, often even to the central portions, depends on the porous nature of its substance.

Streak.—In speaking above of idiochromatic and allochromatic minerals, we have seen that in the former the fine powder of the mineral is also coloured, the colour being charac¬teristic of the mineral. For the purpose of quickly and easily obtaining a mineral in the state of fine powder, it is rubbed on a plate of rough, unglazed, " biscuit " porcelain. The line of powder left upon the plate by the mineral is known as its streak, the colour of which can be easily observed on the white background. The streak is often characteristic of a mineral, and thus the observation of the streak is a step towards the determination of the mineral. The character is not of much practical value in the determination of precious stones, since on account of their hardness they are much more likely to scratch the porcelain than to leave a streak upon it; moreover, the streak of most precious stones, as in other allochromatic minerals, is white, and therefore not a distinguishing feature.