The middle gemstone in the third row of the High Priest's breastplate (Exodus 28:19 and 39:12) is referred to, in Hebrew, as shebu (Strong's Concordance #H7618). Strong's states the word comes from an unused root meaning a "flame" (a subdivision of flashes or streamers) or something that sparkles and translates it as an agate. The Brown-Driver-Briggs Lexicon (BDB) translates this word the same as Strong's. This precious stone is not mentioned as one which God used to adorn Lucifer upon his creation (Ezekiel 28:13) or one that will be used in any of the foundation or gate stones in the New Jerusalem built by the Eternal (Revelation 21:19 - 21).
In all ten major Bibles versions used in this gemstone series, the word shebu was rendered "agate" in Exodus 28:19.
And I will make thy windows of agates, and thy gates of carbuncles, and all thy borders of pleasant stones (Isaiah 54:12, KJV).
Syria was thy merchant by reason of the multitude of the wares of thy making: they occupied in thy fairs with emeralds, purple, and broidered work, and fine linen, and coral, and agate (Ezekiel 27:16, KJV).
Agates, composed of silica with about the same hardness as quartz (6.5 to 7 on the Mohs hardness scale), are one of several varieties of chalcedony stones. They can be colorless or have several layers (bands) of different colors such as white, red, gray and others.
Agates take their name from the river Achates on the island of Sicily, where they were initially found in abundance. They usually occur in volcanic rock deposits or in ancient lava flows. A type of this stone known as the Lake Superior agate (the biggest of the five Great Lakes in the United States), which possesses bands stained by iron, is the official gemstone of the state of Minnesota.
These rocks can be found in modern jewelry such as pins, beads and brooches. In the past, they were used to make cameos.
This precious stone was once believed to make those who wear them persuasive, agreeable, and possess God's favor. It was also thought the stone gave the wearer strength and a bold heart, protected them from all danger, and even made them able to avert lightning strikes. The agate was also believed not only to cure insomnia but also to guarantee good dreams (Curious Lore of Precious Stones, pages 51 - 52)
A 1913 Catholic Encyclopedia article on precious stones says the agate was thought to not only render all poisons as harmless, but also counter contagious diseases and even halt a fever. The stone was even credited with sharpening the vision of those who wore it!
The last gemstone listed in the third row of the High Priest's breastplate (Exodus 28:19 and 39:12) is derived from the Hebrew word achlamah (Strong's Concordance #H306). This word means "dream stone" according to Strong's and it translates it as "amethyst." The BDB states the word likely refers to a purple stone and it also translates achlamah as the same word as 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 in the entire New Testament. It means a rock that is believed to prevent intoxication or make someone "not drunk." Strong's translates this word as "amethyst" and Thayer's lexicon states the word refers to an amethyst which has both a violet and purple color. All ten Bibles used in this series also translate this Greek word as this precious stone.
Anciently, it was believed that any intoxicating liquid drunk from a cup composed of this gem would render the drink unable to cause someone to be drunk. Amulets made of the rock were also worn to counteract the affects of wine. In the Middle Ages, the amethyst was considered a "pious" gem and used by the Church.
In the fifteenth century 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 also was believed to aid those in capturing wild animals (Curious Lore of Precious Stones, page 58).