OPAL
(_White Opal_, _Black Opal_, _Fire-Opal_)
That opal in early times excited keen admiration is evident from
Pliny’s enthusiastic description of these stones: “For in them you
shall see the burning fire of the carbuncle, the glorious purple of the
amethyst, the green sea of the emerald, all glittering together in an
incredible mixture of light.” During much of last century, owing to the
foolish superstition that ill-luck dogs the footsteps of the wearer,
the species lay under a cloud, which has even now not quite dispersed,
but exercises a prejudicial effect upon the fortunes of the stone. It
has, however, recently attracted considerable attention owing to the
discovery of the splendid black opals in Australia; at one moment black
with the darkness of night, at the next by a chance movement glowing
with vivid crimson flame, such stones may justly be considered the most
remarkable in modern jewellery. At the present day opal is divided by
jewellers roughly into two main groups: ‘white’ (Plate XXVII, Fig. 6)
and ‘black’ (Plate XXVII, Fig. 9), according as the tint is light or
dark, fire-opal (Plate XXVII, Fig. 10) standing in a separate category.
Opal differs from the rest of the principal gem-stones in being
not a crystalline body, but a solidified jelly, and it depends for
its attractiveness upon the characteristic play of colour, known,
in consequence, as opalescence (cf. p. 39), which arises from a
peculiarity in the structure. Opal is mainly silica, SiO_{2}, in
composition, but contains in addition an amount of water varying in
precious opal from 6 to 10 per cent. As the original jelly cooled, it
became riddled throughout with cracks, which were afterwards generally
filled with opal matter, containing a different amount of water,
and therefore differing slightly in refractivity from the original
substance. The structure not being quite homogeneous, each crack has
the same action upon light as a soap-film, and gives rise to precisely
similar phenomena; the thinner and more uniform the cracks, the greater
the splendour of the chromatic display, the particular tint depending
upon the direction in which the stone is viewed. The cracks in certain
opals were not filled up, and therefore contain air. Such stones appear
opaque and devoid of opalescence until plunged into water; they are
consequently known as hydrophane, from ὕδωρ, water, and φαίνεσθαι, to
make appear. Owing to the effect of total-reflection, light was stopped
on the hither side of the cracks before they were filled with water,
which is not far inferior to opal in refractivity; it is surprising how
much water these stones will absorb.
Opal is colourless when pure, but is nearly always more or less milky
and opaque, or tinted various dull shades by ferric oxide, magnesia
or, alumina. The so-called black opal is generally a dark grey or
blue, and very rarely quite black. That the coloration is not due
to ordinary absorption, but to the action of cracks in the stone,
is shown by the fact that the transmitted light is complementary to
the reflected light; the blue opal is, for instance, a yellow when
held up so that light has passed through it. In many black opals the
opalescent material occurs in far too tiny pieces to be cut separately,
and the whole iron-stained matrix is cut and polished and sold under
the name ‘opal-matrix.’ The reddish and orange-coloured stones known
as fire-opal have pronounced colour and only slight milkiness; they
display the customary opalescence in certain directions. These stones
are often faceted, but otherwise opals are cut _en cabochon_, either
flat or steep—generally the former in brooches and pendants, and the
latter in rings. Opal is somewhat soft, varying from 5 to 6½ on Mohs’s
scale, and is therefore easily scratched. The specific gravity ranges
from 2·10 to 2·20, and the refractive index from 1·444 to 1·464, the
refraction, of course, being always single. It is unwise to immerse
opals in liquids on account of their porosity.
The name opal comes to us through the Latin _opallus_, which was used
for the same species as understood by the term at the present day, but
the word has a far older origin, which has not been traced. The Romans
also called the mineral _pæderos_, the Greek form of Cupid, a name
applied to all rosy stones. The name cacholong, for the bluish-white
porcelain variety, which is very porous and adheres to the tongue, is
of Tartar origin; the stone is highly valued in the East.
The oldest mines, which up to quite a recent date were the only
extensive deposit of opal known, were at Cserwenitsa, near Kashau,
in Hungary. From them in all probability emanated the opals known to
the Romans. The opals from this locality were generally quite small,
and large pieces were rare and commanded high prices. The Hungary
mines, however, proved quite unable to compete with the rich fields
at White Cliffs, New South Wales, in spite of the efforts that were
made to depreciate and exclude from the market the new stones, and at
the present time few of the opals on the market come from them. As so
often happens, the White Cliffs deposit was discovered by accident.
In 1889 a hunter, when tracking a wounded kangaroo, chanced to pick
up an attractively coloured opal. The district is so waterless and
forbidding that, but for such a chance, the opals might have long lain
hidden. They occur in seams in deposits of Cretaceous Age in a variety
of ways, filling cavities in rocks or sandstones, or cracks in wood, or
replacing wood, saurian bones, and some spiky mineral, which may have
been glauberite. In recent years, another rich deposit was discovered
farther north, on both sides of the boundary between Queensland and
New South Wales. The field is remarkable for the darkness of its
opals, which are called ‘black opal’ in contradistinction to the
lighter-coloured stones previously known. From Lightning Ridge in
New South Wales come stones stained deep black which quite merit the
designation black opal. The sandstone in which they are found is
rich in iron, and this is no doubt responsible for the deepness of
their tint. Mexico is noted for the fire-opal, which is found at
Esperanza, Queretaro, and Zimapan; but other kinds of opal also are
found at these places.
[Illustration: _PLATE XXVIII_
OPAL MINES, WHITE CLIFFS, NEW SOUTH WALES]
The price of opal varies greatly, according to the intrinsic colour
and the uniformity and brilliance of the opalescence. Common opal can
be bought at as low a rate as 1s. a carat, while black opal ranges
from 10s. to £8 a carat; but a good dark stone displaying a flaming
opalescence commands a fancy figure, fine stones of this class being
exceedingly rare. Fire-opal enjoys only a limited popularity now,
though a few years ago it was in some demand; the price runs from 2s.
to 10s. a carat.