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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 (), 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