FLUOR, LAPIS LAZULI, SODALITE, VIOLANE, RHODONITE, AZURITE,
MALACHITE, THULITE, MARBLE, APOPHYLLITE, CHRYSOCOLLA,
STEATITE OR SOAPSTONE, MEERSCHAUM, SERPENTINE
Space will not permit of more than a few words concerning the more
prominent of the numerous mineral species which are employed for
ornamental purposes in articles of virtu or in architecture, but which
for various reasons cannot take rank as gem-stones.
Fluor, a beautiful mineral which is found in its greatest perfection in
England, has enjoyed well-deserved popularity when worked into vases
or other articles. The finest material, deep purple in colour, known
as ‘Blue John,’ came from Derbyshire, but the supply is now exhausted.
The crystallized examples, from Durham, Devonshire, and Cornwall, form
some of the most attractive of museum specimens. The crystals take the
shape of cubes, often twinned, and have an easy octahedral cleavage.
The refraction is single, the index being 1·433. Fluor is noted for its
property of appearing of differing colour by reflected and transmitted
light, and the phenomenon is in consequence known as fluorescence. The
specific gravity is 3·18, and the hardness 4 on Mohs’s scale. Owing to
its low refraction and softness, fluor is not suitable for jewellery.
Clear colourless material is in demand for particular lenses of
microscope objectives.
The lovely blue stone known as lapis lazuli has since the earliest
times been applied to all kinds of decorative purposes, for mosaic
and inlaid work and as the material for vases, boxes, and so on, and
was the original sapphire of the ancients. When ground to powder it
furnishes a fine blue paint, but it has now been entirely superseded
for this purpose by an artificial product. Although to the eye so
homogeneous and uniform in structure, lapis lazuli has been shown
by microscopic examination to be composed of calcite coloured by
three blue minerals in varying proportions. All three belong to the
cubic class of symmetry, and are mainly soda aluminium silicates
in composition; their hardness varies from 5 to 6 on Mohs’s scale.
Lazurite, Na_{4}(NaS_{3}.Al)Al_{2}Si_{3}O_{12}, has specific gravity
varying from 2·38 to 2·45, and hardness about 5 to 5½; haüynite,
(Na_{2},Ca)_{2}(NaSO_{4},Al)Al_{2}Si_{3}O_{12}, is about the same in
specific gravity, 2·4 to 2·5, but slightly harder, 5½ to 6; while
sodalite, Na_{4}(AlCl)Al_{2}Si_{3}O_{12}, is the lightest in density,
2·14 to 2·30, with hardness 5½ to 6, and has a refractive index 1·483.
By far the oldest mines are in the Badakshan district of Afghanistan,
a few miles above Firgamu in the valley of the Kokcha, a branch of
the Oxus, where ruby and spinel are found. It is also found at the
southern end of Lake Baikal, Siberia, and in the Chilian Andes.
Sodalite occurs in beautiful blue masses at Dungannon, Hastings County,
Ontario, Canada, and at Litchfield, Maine, U.S.A. They make excellent
polished stones.
Violane, a massive, dark violet-blue diopside from San Marcel,
Piedmont, Italy, also makes a handsome polished stone.
Rhodonite, silicate of manganese, MnSiO_{3}, possesses a fine red
colour, and makes an attractive stone when cut and polished. It has
very slight biaxial double refraction, the refractivity being about
1·73; the specific gravity is 3·6, and hardness 6. It is found in large
masses near Ekaterinburg in the Ural Mountains, and is quarried as an
ornamental stone.
Both the copper carbonates, azurite or chessylite, and malachite, make
effective polished stones. The latter is also worked into various
ornamental objects; it occurs in fibrous masses, the grained character
of which look well in the polished section. Its colour is a bright
green, to which it owes its name, from μαλακή, mallows. Its composition
is represented by the formula CuCO_{3}.Cu(OH)_{2}, and it is the more
stable form, since azurite is frequently found altered to it. It
has biaxial double refraction, and the indices are about 1·88; the
specific gravity is 4·01, and hardness about 3½ to 4 on Mohs’s scale.
It is found in large masses at the copper mines of Nizhni Tagilsk in
the Ural Mountains, where it is mined as an ornamental stone; it also
accompanies the copper ores in many parts of the world, for instance
Cuba, Chili, and Australia. Azurite, so called on account of its
beautiful blue colour, is rarer, but, unlike malachite, is generally in
the form of crystals. Beautiful specimens have come from Chessy, near
Lyons, France, and Bisbee, Arizona, U.S.A. The composition corresponds
to the formula 2CuCO_{3}, Cu(OH)_{2}. The specific gravity is 3·80, and
hardness about 3½ to 4.
Chrysocolla occurs in blue and bluish-green earthy masses, with
an enamel-like texture, which in some instances can be worked and
polished. Being the result of the decomposition of copper ores, it
varies considerably in hardness, ranging from 2 to 4 on Mohs’s scale.
Its composition approaches to the formula CuSiO_{3}.2H_{2}O, but it
invariably contains impurities. It is very light, the density being
only about 2·2.
Steatite, or soapstone, is a massive foliated silicate of magnesium
corresponding to the formula H_{2}Mg_{3}Si_{4}O_{12}, which is one of
the softest of mineral substances, representing the degree 1 on Mohs’s
scale, but in massive pieces is harder owing to the intermixture of
other substances with it. It has a peculiar greasy feeling to the
touch, due to its softness. The specific gravity is about 2·75. The
Chinese carve images out of the yellowish and brownish pieces.
Meerschaum, a silicate of magnesium corresponding to the formula
H_{4}Mg_{2}Si_{3}O_{10}, is familiar to every smoker as a material for
pipe-bowls. It is very light, the specific gravity being only 2·0, and
soft, the hardness being about 2 to 2½ on Mohs’s scale. When found,
it is pure white in colour, and answers to its name, a German word
signifying sea-foam. It comes from Asia Minor.
Serpentine has been largely used for decorative purposes, as well
as for cameos and intaglios, and formed most of the famous ‘verde
antique.’ Being the result of the decomposition of other silicates
it varies enormously in appearance and characters, but the most
attractive stones are a rich oil-green in colour and resemble jade. The
composition approximates to the formula H_{4}Mg_{3}Si_{2}O_{9}, but it
invariably contains other elements. The hardness varies from 2½ to 4
on Mohs’s scale, according to the minerals contained in the stone; the
specific gravity is about 2·60 and the refractivity 1·570.
The beautiful rose-red stone, thulite, makes a handsome decorative
stone. It has nearly the same composition as epidote (p. 275), and like
it has strong dichroism, the principal colours being yellow, light
rose, and deep rose. The colour is due to manganese. Its refractive
index is about 1·70, specific gravity 3·12, and hardness 6 to 6½ on
Mohs’s scale; it possesses an easy cleavage. Fine specimens come from
Telemark, Norway, and it is therefore called after the old name for
Norway, Thule.
Marble is a massive calcite, carbonate of lime, with the formula
CaCO_{3}. When pure it is white, but it is usually streaked with other
substances which impart a pleasing variety to its appearance. It is
always readily recognized by the immediate effervescence set up when
touched with a drop of acid. Calcite is highly doubly refractive (cf.
p. 40), the extraordinary index being 1·486, and ordinary 1·658, a
difference of 0·172; the specific gravity is 2·71, and hardness 3 on
Mohs’s scale. Lumachelle, or fire-marble, is a limestone containing
shells from which a brilliant, fire-like chatoyancy is emitted
when light is reflected at the proper angle. It sometimes resembles
opal-matrix, but is easily distinguished by its lower hardness and by
its effervescent action with acid. Choice specimens come from Bleiberg
in Carinthia, and from Astrakhan.
Apophyllite has not many characters to commend it, being at the best
faintly pinkish in colour, and always imperfectly transparent. It is
a hydrous silicate of potassium and calcium with the complex formula
(H,K)_{2}Ca(SiO_{3})_{2}.H_{2}O. Its refractivity is about 1·535,
specific gravity 2·5, and hardness 4½ on Mohs’s scale; it possesses
an easy cleavage. It occurs in the form of tetragonal crystals at
Andreasberg in the Harz Mountains, and in the Syhadree Mountains,
Bombay, India.
PART II—SECTION D
ORGANIC PRODUCTS