PRECIOUS STONES. 261
chiefly
in eruptive rocks that have undergone alteration, especially alteration
by deformation. Crystals of Zircon in a pegmatite matrix are shown in
Fig. 34. In composition it is a silicate of zirconium, ZrSi04.
Some interesting changes are seen on heating the mineral; thus a deep
red-brown specimen heated in the dark "will be seen to phosphoresce
suddenly, and it will then be found that the stone is changed to a very
pale yellow colour, or has become colourless altogether; the specific
gravity is increased, and the lustre is rendered greater. Some of these
changes are supposed to be due to chemical alterations in the state of
the iron contained in almost all specimens of the mineral. Even strong
light may cause the stone to change, but the colour is to some extent
restored by keeping the specimen in the dark for a time afterwards.
With the ordinary variety of Zircon we are not much concerned. The gem
variety occurs as Hyacinth, which includes the orange and red-brown
transparent stones, and Jargon, comprising the colourless and pale
yellow specimens.
Most
of the stones suitable for use as gems are found in the gem gravels of
Ceylon, which gravels are derived from the weathered crystalline rocks
in which the Sapphires were developed. The chief localities are
Ratua-pura and Matura, the latter locality giving its name to the "
Matura Diamonds," which are very pale or colourless Zircons. Many other
localities yield the mineral, but rarely in gem quality. Such are:
Bohemia, where it occurs in gneiss; near Unask, in the Ural Mountains;
at Friedriksvarn in Norway in the zircon-syenite; at Mudgee in New
South Wales in the auriferous alluvium ; and in the volcanic districts
of France,
