Prasiolite, also known as Vermarine, Green Amethyst or Greened Amethyst, is a yellow-green to green variety of quartz which can be produced by:
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Amethyst that is naturally heated by lava flows;
Irradiating Amethyst to about 302º Fahrenheit, turning the purple Amethyst to a pale green. However, this method tends to fade to colorless over time. 
Heating Amethyst to about 935º Fahrenheit, turning purple Amethyst to a green or yellowish green color.  This method is how most Prasiolite on the market is produced.