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.
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