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QUARTZ.
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transparent,
and sometimes even shows colours when saturated with water ; hyalite is
transparent ; cacholong is milky and nearly opaque.
Opals are cut with a convex surface ; their brilliancy is often increased by moderate warmth.
Root
of opal contains veins and specks of opal in a dark-coloured
ferruginous matrix, which may be still further darkened by soaking in
oil of vitriol. Cameos are sometimes cut so as to show a head or figure
in precious opal thrown up against a background of the dark-brown
ferruginous matrix of the stone.
Quartz.
The
purest form of quartz is represented by the colourless rock crystal so
largely used for ornamental objects in the cinquecento time, and now
employed extensively for optical purposes. It is silica, the oxide of
silicon (Si02), and contains 47-0 per cent, silicon and 53.0
per cent, oxygen. The coloured varieties contain sometimes very small
traces of foreign matters to which it is presumed their colours are due
: but there are doubts in many cases as to the exact nature of the
causes of these colorations when they are not of merely mechanical or
physical origin. The presence of nickel compounds in the green
chrysoprase, and of titanium in the rose quartz of Rabenstein is
believed to be the cause, in these instances respectively, of the
colours of these two varieties of quartz ; moreover, there is no doubt
that many of the red, green, and brown colours shown by members of this
group are due to manganese and iron oxides, and to silicates of these
metals. Traces of water, alumina, lime, and magnesia also occur, but
these ingredients are of little importance as regards the source of the
hue of different varieties of quartz.
Quartz
crystallises in the rhombohedral system, its commonest form being a
six-sided prism, striated transversely, and terminated by a six-sided
pyramid. In amethyst there are many fine 8445.
G 2
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