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The process of artificially changing or enhancing the colour gemstones, especially a porous stone (e.g. agate), by causing a colouring chemical (or carbon) to enter the pores of the stone, or of imparting a colour to certain colourless stones, in either case sometimes enhanced by heat treatment.
Examples are: banded agate which is dyed various colours; calcite which is then termed Mexican jade; chalcedony which, after being dyed and heated, is cornelian, or when green or blue is sometimes called 'emeraldine' or 'blue moonstone'; onyx which is usually dyed; jasper which dyed blue is sometimes called 'German', 'Swiss' or 'false' lapis lazuli; and chrysoprase which dyed green is sometimes called 'agate'.
If aniline dyes are used to produce varied colours, they will fade in sunlight. The terms 'dyeing' and staining are often used interchangeably.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson