Amethyst
(Heb. achlamah) a subspecies of quartz of a bluish-violet color. Mention is made of this precious stone, which formed the third in the third row of the high priestly breastplate, in Ex 28:19; 39:12 It occurs also in Re 21:20
Bibliography Information Smith, William, Dr "Definition for 'Amethyst' Smiths Bible Dictionary". bible-history.com - Smiths; 1901.