Amethyst is a type of quartz that owes its purple color to background radiation. The latter is primarily from the naturally occurring potassium-40 and members of the uranium and thorium decay series found in rocks and soil. For the radiation to turn quartz purple, the quartz must contain trace amounts of iron. The color change is due to the fact that the radiation exposure results in an oxidation of the iron impurities (Fe+3 to Fe+4) which changes the absorption spectrum.
These three chunks of amethyst were originally part of the same piece. After the original piece was broken into three, the two smaller pieces were packed in sand and decolorized by gradually heating them to 300 degrees C over a 12-hour period. They then looked like the piece in the center. Unfortunately, the heating process also resulted in a cracking of the crystals which left them with a somewhat cloudy appearance. The piece on the right was recolorized by exposing it to gamma rays from cobalt 60.
My amethyst (on the right) isn't of as high a quality as that produced by Nature, but Nature has had a lot more time to practice.
Donated by Paul Frame.