Easton's Bible Dictionary (1897)
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). ” 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.
Smith's Bible Dictionary (1863)
(Heb. achlamah) a subspecies of quartz of a bluish-violet color.
Hastings' Dictionary of the Bible (1898) & Schaff's Bible Dictionary
AM'ETHYST. Ex 39:12; Rev 21:20. A precious stone consisting of crystallized quartz, of a purple or bluish-violet color. Oriental amethyst, a variety of sapphire, is probably included under this latter name.