Amethyst

Amethyst is a purple variety of quartz (SiO2) and
owes its violet color to irradiation, impurities of
iron and in some cases other transition metals,
and the presence of other trace elements, which
result in complex crystal lattice substitutions. Its
name derives from the Greek "amethystos", which means "not drunken", as Amethyst in antiquity
was thought to ward off drunkenness.