Is Amethyst Quartz?
Nearly all quartz contains some trace amounts of iron, on the order of 10-100’s of parts per million. Quartz that contains just the right amount of iron will begin to turn purple and into amethyst if exposed to gamma ray radiation. This low level gamma ray radiation is naturally present in the earth due to nuclear decay in the surrounding rocks. The longer that the iron included quartz is irradiated, the darker the color will be. In fact amethyst can be artificially darkened by exposing it to more gamma radiation.