4	GEMS.
	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 :—