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 XXVI I, 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 fleches 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 aµEOucrvoc, 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.

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 re-fraction, namely, that parallel to the edge of the prism. The ordinary refractive index has the value V544, and the extraordinary r.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.

To physicists quartz is one of the most interesting of minerals because of its power of rotating, to an extent depending upon the thickness of the section, the plane of polarization of a beam of light traversing it in a direction parallel to the prism edge. It appears, moreover, from a study of the pyro electric and general physical characters, that its molecular structure has a helical arrangement, which, like all screws, may have a right- or left-handed character. Amethyst is, in fact, invariably composed of separate twin individuals, alternately right- and left-handed ; in some remarkable crystals the section at right angles to the prism edge is composed of triangular sectors, alternately of different hands and of different tints—purple and white. To the twinning is due the rippled fracture and the feathery inclusions so characteristic of amethyst.

Besides its use for ornamental purposes, quartz finds a place as the material for lenses intended for delicate photographic work, because its transparency to the ultra-violet light is so much greater than that of glass. Spectacle lenses made of it are in demand, because they are not liable to scratches, and retain, therefore, their polish indefinitely. When fused in the oxyhydrogen flame, quartz becomes a silica glass, of specific gravity 2.2 and hardness 5 on Mohs’s scale, which has proved of great service for laboratory ware, because it withstands sudden and unequal heating without any danger of fracture ; it has also in fine threads been invaluable for delicate torsion work, because it acquires not the smallest amount of permanent twist, in this respect being superior to the finest silk threads.

Clear rock-crystal fetches little more than the cost of the cutting ; citrine and amethyst are worth from is. to 5s. a carat, depending upon the quality and size of the stone; smoky-quartz is practically valueless ; rose-quartz realizes less than Is. a carat ; and the value of cat’s-eye is also small—only is. to 2S. 6d. a carat. Tiger’s-eye at one time commanded as much as 25s. a carat, but the supply exceeded the demand, with the consequent collapse in the price.

Beautiful, brilliant, and limpid rock-crystal is found in various parts of the world : in the Swiss Alps, at Bourg d’Oisans in the Dauphine Alps, France, in the famous Carrara marble, in the Marmaros Comitat of Hungary, and in the United States, Brazil, Madagascar, and Japan. Small lustrous stones, known in their localities as ‘ Isle of Wight,” Cornish,’ or ‘ Bristol diamonds,’ are found in our own country. Brazil supplies stones out of which have been cut the clear balls used in crystal-gazing. The finest amethysts come from Brazil—especially the State of Rio Grande do Sul—and from Uruguay, India, and the gem-gravels of Ceylon ; good stones also occur at Ekaterinburg, in the Ural Mountains. A splendid Brazilian amethyst, weighing 334 carats, and two Russian stones—one hexagonal in contour, weighing 88 carats, and the other, a deep purple in colour with a circular table, weighing 73 carats—are exhibited in the British Museum (Natural History). Cairngorm is known from the place of that name in Banffshire, Scotland, whence fine specimens have emanated ; it is a gem much valued in that country. Fine cairngorm has also originated from Pike’s Peak, Colorado. Splendid yellow stones have had their birth in the States of Minas Geraes, Sao Paulo, and Goyaz, of Brazil—especially in the last. The fine Spanish smoky-quartz, which, as already stated, turns yellow on heating, comes from Hinojosa, in the Province of Cordova. The delicate rose-quartz is known at Bodenmais in Bavaria, Paris in Maine, United States, and Ekaterinburg in the Ural Mountains. The finest cat’s-eyes are found in India and Ceylon, and are high in favour with the natives. Greenish stones of an inferior quality are brought from the Fichtelgebirge in Bavaria, and are sold as Hungarian cat’s-eyes,’ despite the fact that no such stone occurs in Hungary—another instance of jewellers’ disdain for accuracy. Tiger’s-eye occurs in considerable quantity in the neighbourhood of Griquatown, Griqualand West, South Africa. A silicified crocidolite, in which the blue colour is retained, comes also from Salzburg, and is known as sapphire- or azure-quartz, or siderite.

Certain of the pebbles found on the seashore of our coasts, especially off the Isle of Wight and North Wales, cut into attractive, clear stones, more or less yellow in colour ; but examples suitable for the purpose are not so numerous as might be supposed, and do not reward any casual search. Les affaires sont les affaires. The local lapidary, instead of explaining that the pebbles brought to him are not worth cutting, finds it more convenient and profitable to substitute for them other, inferior and badly cut, stones, bought by the gross, or even paste stones ; the customer, on the other hand, is contented with a pretty bauble, and is not grateful for the information that it might have been obtained for a fraction of the sum paid.