_Why are the fine crystals of quartz used as a substitute for glass in
spectacles?_

Because, from their superior hardness, they do not so readily become
scratched as glass: they are then termed pebbles.

_Why is the stone Cairn Gorm so called?_

Because it is found in great beauty in the mountain of Cairn Gorm, in
Scotland. It consists of brown and yellow crystals of quartz, and is
much admired for seal stones, &c.; it is sometimes improperly termed
topaz.

_Why is quartz the constituent of so many gems?_

Because the tinges it receives from metals are sufficient to produce
these varieties. Thus, _amethyst_, or purple quartz, is tinged with a
little iron and manganese. _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