Amethyst is the violet and purple varieties of quartz
Amethyst, on heating between 400 C to 500 C, will turn to a
yellow, brown or reddish-brown color, and in some cases will turn
green depending on the original locality of the stone. On heating
above 575 C, amethyst will become colourless and a schiller
effect will set in. Some of the “greened” induced amethyst will
fade when exposed to light for a long period.
Citrine is the yellow variety of quartz.
Sometimes, citrine may duplicate the natural coloring of
amethyst upon irradiation treatment.