Amathyst, is a type of quartz that owes its purple color to background radiation.  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.

The name comes from the Koine Greek αμέθυστος amethystos from α- a-, "not" and μεθύσκω (Ancient Greek) methysko / μεθώ metho (Modern Greek), "intoxicate", a reference to the belief that the stone protected its owner from drunkenness.  Ancient Greeks wore amethyst and carved drinking vessels from it in the belief that it would prevent in toxification.

Amethyst, a semiprecious stone, is often used in jewelry.