CHAPTER XXVIII
QUARTZ
(Rock-Crystal, Amethyst, Citrine, Cairngorm, Cat’s-Eye, Tiger’s-Eye)
ALTHOUGH the commonest and, in its natural form, the most easily recognizable of mineral substances, quartz nevertheless holds a not inconspicuous position among gem-stones, because, as amethyst ([Plate XXVII], Fig. 7), it provides stones of the finest violet colour; moreover, the yellow quartz ([Plate XXVII], Fig. 5) so ably vies with the true topaz that it is universally known to jewellers by the name of the latter species, and is too often confounded with it, and the lustrous, limpid rock-crystal even aspires to the local title of ‘diamond.’ For all purposes where a violet or yellow stone is required, quartz is admirably suited; it is hard and durable, and it has the merit, or possibly to some minds the drawback, of being moderate in price. Despite its comparative lack of ‘fire,’ rock-crystal might replace paste in rings and buckles with considerable advantage from the point of view of durability. The chatoyant quartz, especially in the form known as tiger’s-eye, will for beauty bear comparison with the true cat’s-eye, which is a variety of chrysoberyl. Except that cat’s-eye is cut en cabochon, quartz is step- or sometimes brilliant-cut.
Ranking with corundum next to diamond as the simplest in composition of the gem-stones, quartz is the crystallized form of silica, oxide of silicon, corresponding to the formula SiO2. When pure, it is entirely devoid of the faintest trace of colour and absolutely water-clear. Such stones are called rock-crystal, and it is easy to understand why in early days it was supposed to represent a form of petrified water. It is these brilliant, transparent stones that are, when small, known in many localities as ‘diamonds.’ Before the manufacture of glass was discovered and brought to perfection, rock-crystal was in considerable use for fashioning into cups, vases, and so forth. The beautiful tints characterizing quartz are due to the usual metallic oxides. To manganese is given the credit of the superb purple or violet colour of amethyst, which varies considerably in depth. Jewellers are inclined to distinguish the deep-coloured stones with the prefix ‘oriental,’ but the practice is to be deprecated, since it might lead to confusion with the true oriental amethyst, which is a purple sapphire, one of the rarest varieties of corundum. Quartz of a yellow hue is properly called citrine, but, as already stated, jewellers habitually prefer the name ‘topaz’ for it, and distinguish the true topaz by the prefix Brazilian—not a very happy term, since both the yellow topaz and the yellow quartz occur plentifully in Brazil. Sometimes the yellow quartz is termed occidental, Spanish, or false topaz. Stones with a brownish or smoky tinge of yellow are called cairngorm, or Scotch topaz. The colour of the yellow stones is doubtless due to a trace of ferric oxide. Stones of a smoky brown colour are known as smoky-quartz. Rose-quartz, which is rose-red or pink in colour and hazy in texture, is comparatively rare; strange to say, it has never been found in distinct crystals. The tint, which may be due to titanium, is fugitive, and fades on exposure to strong sunlight. In milky quartz, as the name suggests, the interior is so hazy as to impart to the stone a milky appearance. It has frequently happened that quartz has crystallized after the formation of other minerals, with the result that the latter are found inside it. Prase, or mother-of-emerald, which at one time was supposed to be the mother-rock of emerald, is a quartz coloured leek-green by actinolite fibres in the interior. Specimens containing hair-like fibres of rutile—the so-called flêches d’amour—are common in mineral collections, and are sometimes to be seen worked. When enclosing a massive, light-coloured, fibrous mineral, the stones have a chatoyant effect, and display, when suitably cut, a fine cat’s-eye effect; in tiger’s-eye the enclosed mineral is crocidolite, an asbestos, the original blue hue of which has been changed to a fine golden-brown by oxidation. Quartz which contains scales of mica, hematite, or other flaky mineral has a vivid spangled appearance, and is known as aventurine; it has occasionally been employed for brooches or similar articles of jewellery. Rainbow-quartz, or iris, is a quartz which contains cracks, the chromatic effect being the result of the interference of light reflected from them; it has been artificially produced by heating the stone and suddenly cooling it.
The name of the species is an old German mining term of unknown meaning which has been in general use in all languages since the sixteenth century. Amethyst is derived from ἀμέθυστος, not drunken, possibly from a foolish notion that the wearer was exempt from the usual consequences of unrestrained libations. Pliny suggests as an alternative explanation that its colour approximates to, but does not quite reach, that of wine. Aventurine, from aventura, an accident, was first applied to glass spangled with copper, the effect being said to have been accidentally discovered owing to a number of copper filings falling into a pot of molten glass in a Venetian factory.
Fig. 79.—Quartz
Crystal.
Quartz belongs to the hexagonal system of crystalline symmetry, and crystallizes in the familiar six-sided prisms terminated by six inclined, often triangular, faces (Fig. 79); twins are common, though they are not always obvious from the outward development. In accordance with the symmetry the refraction is double, and there is one direction of single refraction, namely, that parallel to the edge of the prism. The ordinary refractive index has the value 1·544, and the extraordinary 1·553, and since the latter is the greater, the sign of the double refraction is positive. The double refraction is small in amount, but is large enough to enable the apparent doubling of certain of the opposite edges of a faceted stone to be perceptible when viewed with a lens through the table-facet. The dichroism of the deep-coloured stones is quite distinct. Quartz has only about the same amount of colour dispersion as ordinary glass, and lacks, therefore, ‘fire.’ The application of strong heat tends, as usual, to weaken or drive off the colour. Thus the dense smoky-quartz found in Spain, Brazil, and elsewhere is converted into stones of a colour varying from light yellow to reddish brown according to the amount and duration of the application. In the case of amethyst the colour is changed to a deep orange, or entirely driven off if the temperature be high enough. Its density is very constant, varying only from 2·654 to 2·660; the purest stones are the lightest. To it has been assigned the symbol 7 on Mohs’s scale of hardness.