cast, and, when held betwixt the eye and the light, exhibit a changeable
appearance of colour._
_They are always found in a shapeless or massive state, are brittle, and
considerably less hard than most other precious stones._
The only opal mines in the world are those of Hungary. About four
centuries ago, opals were obtained, in such abundance, from these mines,
that upwards of three hundred persons were employed in them. They still
produce opals, some of which are so valuable as to pass, in commerce,
under the appellation of _oriental opals_, whilst others are so poor as
to be of no value whatever to the jeweller. Opals are also found in
other parts of Europe; and in the island of Sumatra and several parts of
the East Indies.
Few precious stones are more beautiful than opals. Their elegant play of
colours, brilliant blue, green, red, and yellow, variously modified, has
procured for them a distinguished rank among gems. Notwithstanding this,
they are but ill suited to the purposes of jewellery, on account of
their softness, their great frangibility, and their sometimes splitting
on a sudden change of temperature. They are usually set without bottoms;
but sometimes with a black bottom, and sometimes with a foil of red,
blue, or gold colour. Their value is such that a fine oriental opal is
considered worth about twice as much as an oriental sapphire of the same
size. By the Turks they are so peculiarly esteemed, that a fine opal of
moderate size has sometimes been sold at the price of a diamond. The
esteem in which they were held among the ancient Romans was such, that
Nonius, the Roman senator, is stated to have preferred banishment to
parting with a favourite opal which Mark Antony was anxious to possess.
In the abbey of St. Denys, near Paris, there was formerly a curious
ancient opal which was green on the outside, and, when viewed against
the light, exhibited a fine ruby colour: and in the Imperial Cabinet at
Vienna, there are two pieces of opal, from the mines in Hungary, one of
which is about five inches long, and 2½ inches broad; and the other the
size and shape of a hen’s egg. Both these stones exhibit a very rich and
splendid play of colours.
In the purchasing of opals great caution is requisite, as fine glass
pastes have not unfrequently been substituted for them, and sold at
enormous prices.
103. HYDROPHANOUS OPAL, _or_ OCULUS MUNDI, _is a kind of opal, the
distinguishing characteristic of which is, that it gradually becomes
transparent, and exhibits a beautiful play of colour after being
immersed in water_. It is either of a whitish brown, yellowish green,
milky grey, or yellow colour, and opaque; and, when touched by the
tongue, adheres to it.
The name of oculus mundi has been given to these stones from an internal
luminous spot, which changes its position according to the direction in
which they are held to the light. The countries in which they are
chiefly found are Hungary and Iceland.
They are sometimes set in rings; and the prices at which they were
formerly valued were, in the highest degree, unaccountable and absurd.
At present their value is considerably lower, though they are still in
great request as objects of curiosity. The phenomenon of their becoming
transparent in water is supposed to be occasioned by that fluid soaking
through their whole substance, in the same manner as the transparency of
paper is occasioned by immersing it in oil. An hydrophanous opal
weighing 27½ grains was kept four minutes in water, and, on being taken
out, weighed 32½ grains, having received in this short period an
augmentation of five grains, or more than one sixth part of its whole
weight. When taken from the water, these stones as they dry become again
opaque.
To preserve them in beauty and perfection, care should be taken not to
immerse them in any but pure water, and to take them out as soon as they
have acquired their full transparency. If these precautions be
neglected, the pores will soon become filled with earthy particles: the
stones will cease to exhibit their peculiar property, and will ever
afterwards continue opaque.
104. COMMON OPAL _is a semi─transparent kind of opal, which does not
exhibit any changeable refraction of colour_. It is found in Germany,
France, Italy, and other countries of the Continent, and is employed for
brooches and other ornaments. A green─coloured Saxon variety is
sometimes cut into ring─stones.
105. MOTHER─OF─PEARL OPAL, _or_ CACHOLONG, _is a milk─white, yellowish,
or greyish─white kind of opal_, which occurs in Iceland, Greenland,
Spain, and the island of Elba. It is sometimes cut into a concave form,
for brooches, and other female ornaments. Italian artists also use it
for mosaic work.
106. WOOD OPAL _appears to be wood that, by some extraordinary operation
of nature, has been converted into opal_. Some specimens exhibit, very
beautifully, the ligneous texture. This kind of opal is chiefly cut into
plates for the tops and bottoms of snuff─boxes. It is found in alluvial
land in some parts of Germany and Hungary. Several years ago the trunk
of a tree, penetrated with opal, and so heavy that eight oxen were
requisite to draw it, was found in Hungary.
PITCHSTONE FAMILY.
107. _OBSIDIAN is a kind of glass, generally of blackish colour, formed
in volcanoes, from which it issues in thick streams._
This substance has been used for various purposes. It is possible to cut
and polish it; but its brittleness and frangibility are so great, that,
without much care, it will fly into pieces during the working. The
reflectors of telescopes are sometimes formed of it. In Mexico and Peru
obsidian is cut into mirrors; and the inhabitants of those countries
used formerly to manufacture it into knives and other cutting
instruments. Hernandez says that he saw more than a hundred of these
knives made in an hour. Cortez, in a letter to the Emperor Charles the
Fifth, relates that he saw razors that had been formed of obsidian. The
natives of Easter and Ascension Islands use this substance for cutting
instruments; and also for pointing their lances and spears, and, in
place of flint, for striking fire with. According to the account that
has been given by Pliny, the ancients sometimes formed obsidian into
mirrors, and ornamental articles of different kinds. The Danish
lapidaries, who obtain considerable quantities of it from Iceland, cut
it into snuff─boxes, ring─stones, and ear─pendants.
Obsidian is found near Mount Hecla, and in other parts of Iceland. Sir
George Mackenzie, during his journey through that island, observed an
immense mass of this substance, which appeared to him to have been part
of a stream that had flowed from a volcano. It is also found in Sicily,
and several other islands of the Mediterranean; and in nearly all parts
of the world where there are volcanoes.
108. _PUMICE is an extremely light and porous mineral, of somewhat
fibrous texture, and of white, grey, reddish, brown, or black colour._
From the texture of this mineral, which is chiefly brought from the
neighbourhood of volcanoes, some persons have imagined it to be asbestos
decomposed by the action of fire. Its lightness is such that, placed in
water, it will float.
To mechanics and other artists pumice stone is a very useful mineral. It
is employed for cleansing and smoothing the surface of wood, leather,
metal, stones, glass, and other substances; and it is used by
parchment─makers, curriers, and hat─makers. Hence it forms a
considerable article of trade: and is exported from the Lipari Islands,
in great quantities, to the different countries of Europe. Sailors in
the Mediterranean rub their beards off with pumice, instead of shaving.
On account of its porosity, it is used in Teneriffe as a filtering
stone. It forms a pernicious ingredient in some kinds of tooth powder;
and in Italy is ground and used instead of sand, in the making of
mortar. Pumice occurs in Ireland, along with obsidian (107); and it
abounds in several islands of the Grecian Archipelago.
AZURE STONE FAMILY.
109. _LAPIS LAZULI, AZURE STONE, or LAZULITE, is a mineral of azure─blue
colour in various shades, and generally accompanied with white or
clouded spots, and also with pyrites (236), which have the appearance of
golden veins or spots. Its texture is earthy, and fracture uneven. It is
opaque, or nearly so, and, in some parts, is sufficiently hard to strike
fire with steel. We are not informed that lapis lazuli is otherwise
found than in shapeless masses or lumps._
About fifty years ago this stone was an article much in fashion for
various ornamental parts of dress. Being capable of very high polish, it
was cut into beads, stones for rings, bracelets, and necklaces. It was
also cut into ornamental vases, small statues, and the tops and bottoms
of snuff─boxes; but of late it has been almost wholly out of use for
these purposes. Before the French Revolution it was imported, to
considerable extent, into that country from the Persian Gulf for the
inlaying of richly─decorated altars; and its value was appreciated
according to the proportion of its yellow spots or veins: these, by many
persons, were erroneously considered to be of gold.
The most important purpose to which lapis lazuli is now applied is in
the manufacturing of the beautiful and brilliant blue colour so much
esteemed by painters, called _ultramarine_. For the making of this, such
pieces are selected as contain the greatest proportion of blue
substance, and consequently the least yellow or white. These are burned
or calcined, reduced to a fine powder, made into a paste with wax,
linseed oil, and resinous matters of different kinds, and afterwards
separated by washing. The powder that is left in this operation, which
requires much time and great attention to perform, is ultramarine.
There are few colours so little susceptible of change from the effects
of time as ultramarine: the consequence of this has been that, as
several of the ancient painters introduced it for the representation of
blue drapery, their pictures, in many instances, are now devoid of
harmony, as this colour alone has stood, whilst all the others have
changed.
Lapis lazuli is principally brought from Persia, Natolia, and China; but
it is also found in Siberia and Tartary. In Europe it has been
discovered only in Germany, and among the ruins of Rome.
A coarse imitation of it is sometimes made by throwing copper filings
into blue enamel whilst in a melted state.
FELSPAR FAMILY.
110. _COMMON FELSPAR is a hard kind of stone which varies much in
colour, being flesh─red, bluish grey, yellowish white, milk─white, or
brownish yellow._
_It is found in a massive state, disseminated or crystallized in four,
six, and ten─sided prisms; will strike fire with steel, and is sometimes
opaque and coloured, sometimes transparent and whitish._
The name of felspar is derived from the German language, and signifies
_spar of the fields_. It is a very common substance, and constitutes a
principal part of many of the highest mountains of the world. When
exposed to weather, it gradually acquires an earthy appearance, and at
length passes into porcelain clay (118).
Felspar is of great use in the manufacture of the finer kinds of
earthenware. Of the two substances which chiefly compose the porcelain
of China, one called _petunzé_ is a whitish laminar kind of felspar.
This mineral is used in the celebrated porcelain that is manufactured at
Sevres, near Paris, for the purpose of giving to it a white and
transparent appearance. Previously to being used, it is pulverized, made
into a paste, and suffered to dry. It is sometimes applied to the
surface of ornamental vases in the form of enamel.
111. AMAZON STONE _is a green variety of felspar_, which is found in
small rolled pieces on the bank of the river of Amazons, in South
America, whence it has its name. It is susceptible of a beautiful
polish, and is often cut into ring─stones, brooches, and the tops of
snuff─boxes. Lapidaries consider it to be most estimable when
accompanied by mica, which gives it a kind of speckled perlaceous
appearance.
112. _LABRADOR FELSPAR is a very beautiful stone, of smoky grey colour,
intermingled with veins and shades of blue, green, and golden yellow,
exhibiting a brilliant play of_ _colours, according to the position with
respect to the light in which it is held._
The original discovery of this singular mineral was by the Moravian
missionaries, on the island of St. Paul, near the coast of Labrador; but
it has since been found in various parts of Norway and Siberia. Persons
who have passed in boats along the rivers of Labrador, have described
the extremely brilliant and beautiful appearance which the rocks of this
substance frequently exhibit in shallow places, at the bottom of the
water. The visitors of the late Leverian Museum will, no doubt,
recollect a remarkably fine mass of Labrador felspar, the surface of
which was polished, and exhibited some of the most splendid and
beautiful colours that can be imagined. It was considered to have been
the most capital specimen that was ever brought to England.
This mineral, on account of its hardness, its brilliancy, and its
capability of receiving a high polish, is in considerable estimation
among lapidaries for different kinds of ornamental work, particularly
for the tops and bottoms of snuff─boxes, for brooches, and necklaces.
113. _MOONSTONE, or ADULARIA, is the purest kind of felspar that is
known; and is considered to have the same relation to common felspar
that rock crystal has to common quartz. Its colour is white, sometimes
with a shade of yellow, red, or green._
The translucent varieties of this stone, when viewed in a certain
direction, sometimes exhibit a pearly and silvery play of colour. These
are valued by jewellers, who cut them into a semi─globular form, and
sell them under the name of moonstone. Those specimens are considered
most estimable which, when cut in a very low oval, present the silvery
spot in the centre of the stone. They are generally used for rings and
brooches; and when set round with diamonds, their pearly lustre exhibits
a striking and agreeable contrast with the brilliancy of that gem.
Adularia is said to have been first discovered by an Italian
mineralogist, near Mount St. Gothard, in Switzerland. He named it
Adularia felspar, in the belief that the mountain on which he had found
it was named Adula. This, however, was not the case; for Mount Adula is
at some distance from St. Gothard, in the Grisons. This mineral has
since been found in the granite of the island of Arran, in France, and
Germany. The finest specimens are brought from Ceylon.
───────
ORDER II.–SOFT STONES.
(Those which will not scratch Glass.)
───────
CLAY FAMILY.
114. Clay is a mixture of alumine (33) and silex (38), and is too well
known to require much description.
It is opaque, has an earthy texture, is about twice as heavy as water,
when moistened is very ductile, adheres slightly to the tongue; and with
its peculiar smell (called clayey) every one is acquainted.
115. _COMMON CLAY, or POTTER’s CLAY, which is found in nearly every
country of the world, is sometimes white, has a blue or yellowish tinge,
or is brown or reddish._
It is the peculiar quality of this substance to become so hard by heat
that it will even strike fire with steel. The ductility of clay, and its
property of thus hardening in the fire, have rendered it an article of
indispensable utility to mankind in all civilized countries. It is
formed into eating vessels of almost every description; plates, dishes,
cups, basins, bowls, and pans for keeping provisions in. For these
almost any kind of clay may be advantageously used; but it is necessary
to mix it with sand, for the purpose of rendering the vessels that are
made of it more firm and strong. Those that are applied to culinary, and
other uses in which it is requisite for them not to be penetrable by
water, are covered with a glazing. This glazing, for coarse ware, is
sometimes made with lead, and sometimes by throwing a certain portion of
salt into the furnace. In the formation of the better kinds of
earthenware, the clay is made into a paste with water, moulded into the
requisite shape upon an horizontal wheel, the inside being formed by one
hand of the potter, and the outside by the other, as the wheel turns
round. When the pieces have been baked, they are dipped into a glazing
mixture, consisting of white lead, ground flints, and water, and are
exposed a second time to the fire. The different colours of earthenware
are obtained by means of various kinds of metallic oxides (21).
The coarser kinds of clay are manufactured into _bricks_ for the
building of houses, and _tiles_ for the covering and paving of them.
These are formed in moulds of the requisite shape, afterwards dried for
some time in the sun, and finally piled in kilns, and there baked to a
proper degree of hardness. The earth for bricks ought to be sufficiently
fine, free from pebbles, and not too sandy, which would render them
heavy and brittle; nor ought it to be entirely free from sand, as this
would make them crack in drying.
Clay is a substance of inestimable value for forming the bottoms of
ponds, and the bottoms and sides of canals and reservoirs, to prevent
the water from draining away. It also composes, in a great measure,
those tenacious earths called arable soils. What is peculiarly
denominated clay land is known by its holding water, and not soon drying
when wetted. Such land requires much labour from the husbandman, before
it can be sufficiently pulverized, or brought to a fit state for being
productive of corn or grass.
116. PIPE CLAY _is a fine and yellowish white variety of common clay_.
It is very plastic, adheres strongly to the tongue; and, in a strong
heat, is hardened, and rendered perfectly white.
It is of this clay that tobacco pipes are made, by the simple process of
casting them in moulds, forming a hole through the stems by means of a
wire, generally dipping the small end into some glazing material, and
then baking them. Pipe clay is also formed into oblong pieces, dried,
and employed for cleaning white woollen cloths, and for various purposes
of domestic utility. It is likewise the basis of the yellow, of what is
called _Queen’s ware_ pottery. This is glazed in a manner somewhat
different from that of common pottery. The glazing mixture consists of a
certain proportion of carbonat of lead (239), ground flint, and flint
glass, worked with water to the thickness of cream. The ware, before it
is glazed, is baked, and thus acquires the property of strongly imbibing
moisture. It is then dipped into the above composition; exposed a second
time to the fire, by which the glaze it has imbibed is melted. A thin
glossy coat is thus formed upon its surface, which is more or less
yellow, according to the greater or less proportion of lead that has
been used.
117. _LOAM is a yellowish or brownish kind of clay; sometimes containing
a considerable proportion of sand. It occurs in immense beds, and is
found in almost every part of the world._
This substance, when mixed with straw or hair, to prevent it from
cracking, is extensively used for the building of what are called mud
cottages or houses. These are generally reared on a foundation of stone,
or brickwork, to secure them from injury by the moisture of the earth.
It is said to be the most advantageous practice to form the loam into
bricks, and to dry these in the shade, and afterwards in the sun. The
use of such bricks is of great antiquity. We are informed that the
ancient city of Damascus, and even the walls of Babylon, were
constructed of bricks made of loam.
118. _PORCELAIN CLAY is generally of white or reddish white colour,
sometimes inclining to yellowish or grey. When dry, it absorbs moisture
rapidly; and it becomes very tenacious when kneaded._
_It is known from the other clays by the fineness of its particles, its
soiling the fingers much when handled, and its fine but meagre feel._
The usual distinction betwixt earthen ware and porcelain is, that the
former is opaque, and the latter semi─transparent. In the manufacture of
porcelain the clay is sometimes used alone, and sometimes intermixed
with other earths, or with felspar (110). The earliest manufacture of
porcelain is supposed to have been that in China and Japan. The quantity
produced in China must formerly have been extremely great; as not only a
considerable portion of the eastern parts of the world, but almost the
whole of Europe, was supplied with it. In a single province it is said
that nearly a million of persons were at one time employed in this
manufacture.
The manufactory at Sevres, in France, has long been celebrated both for
the excellence and elegance of its porcelain. There are well─known
manufactories of porcelain at Meissen in Saxony, at Berlin, and in
Austria; but none of these are at present superior to our own, in
Worcestershire and Staffordshire.
Porcelain clay occurs chiefly in countries which abound with granite
(251) and gneiss (255). It is found in small quantity in Cornwall, and
other granite districts of England, as well as in those of Scotland and
Ireland. But the most valuable kinds of this clay are found in China and
Japan.
The mineral is not used in the state in which it is found in the earth;
but is previously washed several times to free it from impurities. After
the process of washing, only about fifteen parts of pure clay remain:
this is the _kaolin_ of the Chinese. To form the composition of the
porcelain, this clay is mixed, in certain proportions, with quartz (76),
flint, gypsum (192), steatite (124), or other substances; and the
mixture is sifted several times through hair sieves. It is afterwards
moistened with rain water, and, in the form of a paste, is put into
covered casks. Here a fermentation soon takes place, which changes its
smell, colour, and consistence. Its colour passes from white into dark
grey; and the matter becomes both tougher and more soft than before. The
peculiar mode of preparing this mixture, and the art of rightly managing
it, are secrets in most porcelain manufactories.
The next operation consists in giving to the paste thus formed the
requisite shape of the vessels. This is done first by kneading it with
the hands; and then by taking up certain portions of it, and turning it
on a lathe, in the manner of common pottery (115), but with more care.
The third operation is the baking or firing. This is done in furnaces of
a particular construction, and generally lasts from thirty─six to
forty─eight hours. The state of the baking is shown by proof pieces, as
they are called, which are placed in convenient situations, and can be
drawn out, from time to time, for examination. The porcelain in this
state, is named _biscuit porcelain_; and figures, and such other
porcelain articles as are neither to be painted nor exposed to water are
in the state of biscuit.
A fourth operation is covering the surface of the biscuit with a varnish
or enamel. This is composed of pure white quartz (76), white porcelain,
and calcined crystals of gypsum (192); and sometimes principally of
felspar (110). These substances are carefully ground, then diffused
through water, and formed into a paste. When used, the paste is diluted
in water, so as to give it considerable fluidity; and the pieces of
biscuit porcelain are separately plunged into it, in such manner as to
cover their whole surface. These are then exposed to a heat sufficient
to melt the enamel or covering: and in this state they constitute white
porcelain.
If the porcelain is to be painted, it must again be exposed to heat in
the furnace. The colours used for the painting of it are all derived
from metals; and many of them, though dull when applied, acquire
considerable lustre by the action of the fire. The colours are always
mixed with some kind of flux, such as a mixture of glass (204), borax
(208), and nitre, melted together, and afterwards ground.
Gum or oil of lavender is used for mixing up the colours. When the
painting is finished, the pieces are exposed to a heat sufficient to
melt the flux, and thus fix the colour.