Alamandine garnet and purple amethyst spectra

Cut, tear-drop alamandine garnet (top) and similarly cut amethyst (below). These represent quite different colouring mechanisms. The amethyst, basically quartz with impurities of iron and aluminium, can be affected by irradiation to produce 'colour centres' resulting, in this case, in a purple stone. The garnet (Fe_3Al_2(SiO_4)_3) is coloured by ferrous iron: its rather complex and beautiful visible absorption spectrum (red line) was first described by A. Church in 1866. This is one of the easiest gemstones to identify from its spectrum alone.

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