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The process of changing or eliminating the colour of a natural gemstone or a synthetic gemstone by the application of controlled heat, due to a rearrangement of the atoms of the tinctorial agent.
The changes are not in all cases permanent. The process of changing the colour of a metal by heating. Examples are: gold which changes to reddish when the presence of the copper in a low-carat alloy sometimes results in a varicoloured effect; copper which changes as a layer of cuprous oxide forms, going through yellow, bronze, light blue, and dark blue.
From: An Illustrated Dictionary of Jewelry, autor: Harold Newman, publishers: Thames and Hudson