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.Examples are:an amethyst which, when heated, becomes yellow (and resembles citrine) or green, and when more highly heated becomes colourlessa yellow topaz which changes to rose-pinka zircon of reddish-brown colour which changes to blue or yellow or becomes colourlessa dark quartz (morion) which becomes brown and then colourlessan aquamarine or beryl of greenish-yellow colour which changes to blueThe 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-caratalloy 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.