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 ofimparting 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 sometimescalled '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 sometimescalled 'agate'.If aniline dyes are used to produce varied colours, they will fade in sunlight. The terms 'dyeing' and staining are often used interchangeably.