Antique jewelry glossary

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Heat treatment

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 colourless
• a yellow topaz which changes to rose-pink
• a zircon of reddish-brown colour which changes to blue or yellow or becomes colourless
• a dark quartz (morion) which becomes brown and then colourless
• an aquamarine or beryl of greenish-yellow colour which changes to blue

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