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.
