Amethyst

one of the precious stones in the breastplate of the high priest
(Ex. 28:19; 39:12), and in the foundation of the New Jerusalem
(Rev. 21:20). The ancients thought that this stone had the power
of dispelling drunkenness in all who wore or touched it, and
hence its Greek name formed from "a", "privative," and "methuo",
"to get drunk." Its Jewish name, "ahlamah'", was derived by the
rabbins from the Hebrew word "halam", "to dream," from its
supposed power of causing the wearer to dream.

It is a pale-blue crystallized quartz, varying to a dark
purple blue. It is found in Persia and India, also in different
parts of Europe.