The Tourmaline.
The tourmaline is a most complex substance; almost every stone obtained has a different composition, some varying but slightly, with mere traces of certain constituents which other stones possess in a perceptible degree. Consequently, it is not possible to give the chemical formula, which might, and possibly would, be found but seldom, even in analyses of many specimens. It will therefore be sufficient to state the average composition, which is:—ferrous oxide, manganous oxide, potash, lime, boracic acid, magnesia, soda, lithia, and water. These form, roughly speaking, 25 per cent. of the bulk, the remainder being oxide of silicon and oxide of aluminium in about equal parts. It crystallises in the 2nd (hexagonal) system, with difficult cleavage and vitreous lustre.
It will naturally be expected that a substance of such complexity and variety of composition must necessarily have a corresponding variety of colour; thus we find in this, as in the corundum, a wonderful range of tints. The common is the black, which is not used as a gem. Next come the colourless specimens, which are not often cut and polished, whereas all the transparent and coloured varieties are in great demand. To describe adequately their characteristics with relation to light would alone require the space of a complete volume, and the reader is referred to the many excellent works on physics (optics) which are obtainable. This stone is doubly refracting, exhibiting extremely strong dichroism, especially in the blue and the green varieties. It polarises light, and when viewed with the dichroscope shows a remarkable variety of twin colours. It will be remembered that in Hogarth's "Rake's Progress," the youth is too engrossed in the changing wonders of a tourmaline to notice the entrance of the officers come to arrest him.
CHAPTER XV.
VARIOUS PRECIOUS STONES—continued.
Zircon.
Zircon appears to have been first discovered by Klaproth in 1789, in the form of an earth, and six years later he found that the stone hyacinth contained a similar substance, both having the formula, ZrSiO4, and both having as their colouring agent ferric oxide. There are several methods of obtaining the metallic element, zirconium; it is however with the silicate of zirconium that we have to deal at the moment. This is called zircon, ZrSiO4, or hyacinth when transparent or red, but when smoke-coloured, or colourless, it is the jargoon, or jarcon, and is found in silt and alluvial soils, limestone, gneiss, and various forms of schist, in India, Australia, the Urals, and certain parts of America. It is often combined with and found in juxtaposition to gold and certain varieties of precious stones. The lines of cleavage are parallel to the sides of the prism, and the crystals have an adamantine, or diamond lustre, varying from the completely opaque to the transparent. In some varieties the oxide of uranium is also present in traces. It crystallises in the 3rd (tetragonal) system, with indistinct cleavage. Its specific gravity varies from 4.70 to 4.88, according to the specimen and the locality.
This stone, like some of the others described, has a very wide range of colour, going through reds, browns, greens, yellows, oranges, whites, greys, blues from light to indigo, notwithstanding which it is somewhat difficult to imitate scientifically, though its composition of 33 per cent. of silica with 67 per cent. of zirconia (the oxide of zirconium), is practically all it contains, apart from the colouring matter, such as the metallic oxides of iron, uranium, etc. Its hardness is 7-1/2, consequently it is untouched by a file, and so far, if one or perhaps two of the three qualities of colour, hardness, and specific gravity, are obtained in a chemically made zircon, the third is wanting. Under the blowpipe, zircons are infusible, but the coloured stones when heated strongly become heavier, and as they are contracting, their colour fades, sometimes entirely, which changes are permanent, so that as they possess the adamantine lustre, they are occasionally cut like a diamond, and used as such, though their deficiency in fire and hardness, and their high specific gravity, make them readily distinguishable from the diamond.