Amethyst may turn brownish or red at a temperature between 400 and 500°C, but sometimes a green color is produced and such gems are sold as "greened amethyst," or "Prasiolite." Further heating causes a complete loss of color. The heating of amethyst to a brownish-yellow color is carried out on a commercial scale. The resulting material is often sold as "Madeira topaz," a misleading name that should be abandoned.

A lighter shade has been called "Palmyra topaz," and reddish stones "Spanish topaz." The color change of amethyst due to heating is not always predictable and fading is a possibility.

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The most commonly heat-treated gem is quartz. Synthetic amethyst is made from a specially prepared smoky quartz by gamma-ray bom¬bardment. It appears likely that natural amethyst acquires its color in the same manner. Gamma irradiation plus heating of some Brazilian quartz produces a bright greenish-yellow color not found in nature. This color fades substantially in sunlight.