colored varieties of quartz
Ironcontaining varieties: Amethyst Large geodes of amethyst occur in basalts in southern Brazil and northern Uraguay. The color of amethyst is the result of radiation damage to Fe 3+ in the interstitial site of quartz. The radiation could be due to gamma rays from 40 K. Current theory says that the ferric iron is oxidized to Fe 4+ by the gamma rays. ...