Amethyst is the last gemstone listed in the third row of the High Priest's breastplate (Exodus 28:19 and 39:12). This Biblical word is derived from the Hebrew word achlamah (Strong's Concordance #H306) and means "dream stone" according to Strong's Exhaustive Concordance. The BDB states the word likely refers to a purple stone. It also translates achlamah as amethyst just like Strong's.
The twelfth foundation gemstone used in the New Jerusalem (Revelation 21:20) is called in the Greek amethustos (Strong's #G271), which is used only in this verse. It means a rock that is believed to prevent intoxication or make someone "not drunk." Strong's and Thayer's Lexicon translates this word as "amethyst," with Thayer's adding it has both a violet and purple color.
This gemstone is a variety of quartz that is violet (purple) colored. It was highly prized by the ancient Egyptians.
Anciently, it was believed that any intoxicating liquid drunk from a cup composed of amethyst would render the drink unable to cause someone to be drunk. Amulets made of the rock were also worn to counteract the effects of wine. In the Middle Ages, it was considered a pious gem and used by the Church.
In the fifteenth century A.D., amethyst was believed to control evil thoughts, make a person generally smarter and give them the ability to be a shrewd in business. It also, according to legend, gave soldiers the ability not to be harmed in battle and be victorious over their enemies. It was also believed to aid those in capturing wild animals (Curious Lore of Precious Stones, page 58).