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Gems - Ornamental Stones

( Originally Published 1919 )


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 When ground to powder it furnishes a fine blue paint, but it has now been entirely superseded for this purpose by an artificial product.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, Na4(NaS3.Al)Al2Si3O12, has specific gravity varying from 2.38 to 2.45, and hardness about 5 to 5 ; hauynite, (Na2,Ca)2(NaSO4,Al)Al2Si3O12, is about the same in specific gravity, V4 to 2.5, but slightly harder, 51 to 6 ; while sodalite, Na4(AICl)AI2Si3O12, is the lightest in density, 2. I 4 to 2.30, with hardness 5 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 0f 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, MnSiO3, 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, mallows. Its composition is represented by the formula CuCO3.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 3 1/2 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 2CuCO3, Cu(OH)2. The specific gravity is 3.80, and hardness about 3 1/2 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 CuSiO3.2 H2O, 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 H2Mg3Si4O12, 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 27 5. The Chinese carve images out of the yellowish and brownish pieces.

Meerschaum, a silicate of magnesium corresponding to the formula H4Mg2Si3Olo, 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 2 1/2 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 H4Mg3S12O9, but it invariably contains other elements. The hardness varies from 2 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 t 70, specific gravity 3.12, and hardness 6 to 61 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 CaCO3. 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 r658, a difference of 0.172 ; the specific gravity is 2.7 t, 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)2Ca(SiO3)2.H2O. Its refractivity is about r535, specific gravity 2.5, and hardness 41 on Mohs's scale ; it possesses an easy cleavage. It occurs in the form of tetragonal crystals at Andreas-berg in the Harz Mountains, and in the Syhadree Mountains, Bombay, India.

For further information about Ornamental Stones:
Ornamental Stones @ Wikipedia

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