_Rose quartz_, or false ruby, derives its
colour from manganese. _Avanturine_ is a beautiful variety of quartz, of
a rich brown colour, which, from a peculiarity of texture, appears
filled with bright spangles. Small crystals of quartz, tinged with iron,
are found in Spain, and have been termed _hyacinths of Compostella_.
Flint, chalcedony, carnelian, onyx, sardonyx, and bloodstone, or
heliotrope, and the numerous varieties of agates, are principally
composed of quartz, with various tinging materials.
_Why is opal among the most beautiful productions of the mineral world?_
Because the colours are not occasioned by any particular tinge of the
substance, but by its peculiar property of refracting the solar rays. It
is a compound of about 90 silica, and 10 water. The finest specimens
come exclusively from Hungary. There is a variety of opal called
_Hydrophane_, which is white and opaque till immersed in water; it then
resembles the former.
_Why is the sapphire genus so highly prized?_
Because, after diamond, it is the hardest substance in nature. It forms
also the most valuable gems, as the oriental ruby and the topaz. The
blue variety, or sapphire, is harder than the ruby.
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