the
varieties of amethyst, rock crystal, plasma, agate, jaspers, obsedian,
and opal ; amongst the fluorites is the topaz ; amongst the aluminous
silicates are the species of garnet, emerald, felspar, and porphyry.
Gems
are found naturally under two distinct modes of composition. Some have
the atoms arranged evidently according to a fixed law, and are called
" crystalline gems," crystallized gems and crystals. Others have very
minute particles or molecules simply collected together, without
constant regularity of form, and are called amorphous gems, from the
Greek a, " without," μορφή, " form ;" all the gems that
solidify, after having heen in a fluid state, belong to this class :
when they are found under condensed gelatinous forms like opals they
are called porodine ; when they appear to have been condensed by ignition into vitrifications they are called "jaline," i.e., vitreous
crystal; this in mineralogy denotes a solid body, which presents
naturally, and without the assistance of art, a many-sided figure, more
or less regular, which varies almost infinitely. However, it must be
remarked that these can be divided into six very distinct groups or
systems, as follow :—
