The Varieties Of Silicates Stone

The Varieties Of Silicates Stone


The chief of these are the garnets, crystallising in the cubic system, and anhydrous. The garnet is usually in the form of a rhombic dodecahedron, or as a trisoctahedron (called also sometimes an icosatetrahedron), or a mixture of the two, though the stones appear in other cubic forms. 
In hardness they vary from 6-1/2 to 8-1/2. 
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They average from 40 to about 42 per cent. of silica, the other ingredients being in fairly constant and definite proportions. 
They are vitreous and resinous in their lustre and of great variety of colour, chiefly amongst reds, purples, violets, greens, yellows and blacks, according to the colouring matter present in their mass. 

There are many varieties which are named in accordance with one or more of their constituents, the best known being 
the variety of silicate stone

(A) The iron-alumina garnet, having the formula 6FeO, 3SiO2 + 2Al2O3, 3SiO2. 
This is the "precious" garnet, or almandine, sometimes called the "Oriental" garnet; these stones are found in Great Britain, India, and South America, and are deep red and transparent, of vitreous lustre. 
They get up well, but certain varieties are so subject to defects in their substance, brought about by pressure, volcanic action, and other causes, some of which are not yet known, that their quality often becomes much depreciated in consequence. 
This inferior variety of the iron-alumina garnet is called the "common" garnet, and has little lustre, being sometimes opaque. 
The perfect qualities, or almandine, as described above, are favourite stones with jewellers, who mount great quantities of them. The second variety is the 

(B) lime-iron garnet, formula, 6CaO,3SiO2 + 2Fe2O3,3SiO2. 
The chief of this class is the melanite, sometimes dull, yet often vitreous; it is mostly found in volcanic rocks, such as tuff; this variety is very popular with jewellers for mourning ornaments, for as it is a beautiful velvet-black in colour and quite opaque, it is pre-eminent for this purpose, being considerably less brittle than jet, though heavier. 
Another variety is the "topazolite," both yellow and green. 
The "aplome" is greenish-yellow, yellowish-green, brown, and usually opaque. A further form of lime-iron garnet is the "pyreneite," first found in the Pyrenees Mountains, hence its name. The 

(C) lime-chrome garnets—6CaO,3SiO2 + 2Cr2O3, 3SiO2—the chief of which is "uwarowite." 
This is of a magnificent emerald green colour, translucent at edges[Pg 102] and of a vitreous lustre. 
When heated on the borax bead it gives an equally beautiful green, which is, however, rather more inclined to chrome than emerald. 
This is an extremely rare stone in fine colour, though cloudy and imperfect specimens are often met with, but seldom are large stones found without flaws and of the pure colour, which rivals that of the emerald in beauty. The fourth variety 

(D) is the lime-alumina garnet, its formula being—6CaO,3SiO2 + 2Al2O3,3SiO2. 
Like the others, it has a number of sub-varieties, the chief being the "cinnamon stone," which is one of great beauty and value when perfect. 
This stone is almost always transparent when pure, which property is usually taken as one of the tests of its value, for the slightest admixture or presence of other substances cloud it, probably to opacity, in accordance with the quantity of impurity existent. 
This variety is composed of the oxides of aluminium and silicon with lime. 
In colour it ranges from a beautiful yellowish-orange deepening towards the red to a pure and beautiful red. "Romanzovite" is another beautiful variety, the colour of which ranges through browns to black. Another important variety is the "succinite," which gets up well and is a favourite with jewellers because of its beautiful, amber-like colour, without possessing any of the drawbacks of amber.

(E) The magnesia-alumina garnet—6MgO,3SiO2 + 2Al2O3,3SiO2—is somewhat rare, the most frequently found being of a strong crimson colour and transparent. 
This variety is called "pyrope," the deeper and richer tints being designated "carbuncle," from the Latin carbunculus, a little coal, because when this beautiful variety of the "noble" garnet is held up between the eyes and the sun, it is no longer a deep, blood-red, but has exactly the appearance of a small piece of live or glowing coal, the scarlet portion of its colour-mixture being particularly evident. 
The ancient Greeks called it anthrax, which name is sometimes used in medicine to-day with reference to the severe boil-like inflammation which, from its burning and redness, is called a carbuncle, though it is more usual to apply the word "anthrax" to the malignant cattle-disease which is occasionally passed on to man by means of wool, hair, blood-clots, etc., etc., and almost always ends fatally. 
A great deal of mystery and superstition has always existed in connexion with this stone—the invisibility of the bearer of the egg-carbuncle laid by a goldfinch, for instance. 

(F) The manganese-alumina garnet—6MnO,3SiO2 + 2Al{2}O3,3SiO2—is usually found in a crystalline or granular form, and mostly in granite and in the interstices of the plates, or laminæ, of rocks called schist. 
One variety of this, which is a deep hyacinth in colour, though often of a brown-tinted red, is called "spessartine," or "spessartite," from the district in which it is chiefly found, though its distribution is a fairly wide one. 

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