Precious stones, such as the diamond, ruby, topaz, and emerald. Imitations of them consist of glasses, but the ruby and the sapphire are now produced commercially by the fusion of pure alumina, and are identical in all respects with the natural gems. The colour of the ruby is due to a small content of chromium, as also probably that of the sapphire; the emerald and aquamarine owe their green tint to the presence of ferrous iron, and are varieties of beryls; the amethyst and the garnet also owe their colours to ferric iron or manganese, or both. The individual gems are further referred to under their separate names.
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