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 SiO_{2}. 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.
[Illustration: 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.
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 1s. to 5s. a carat, depending
upon the quality and size of the stone; smoky-quartz is practically
valueless; rose-quartz realizes less than 1s. a carat; and the value
of cat’s-eye is also small—only 1s. 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 Dauphiné
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, São
Paulo, and Goyãz, 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.