aturally 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, ZrSiO_{4}, 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, ZrSiO_{4}, 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,