Agates can be found in the middle gemstone in the third row of the High Priest's breastplate (Exodus 28:19 and 39:12). It is referred to, in Hebrew, as shebu (Strong's Concordance #H7618). Strong's states the Bible 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 as agates as well.
Agates are not mentioned as one of the gems which God used to adorn Lucifer (later, Satan the devil) upon his creation (Ezekiel 28:13). It is also not mentioned in the Bible in relation to any of the foundation or gate stones in the New Jerusalem (Revelation 21:19 - 21).
In all major Bible versions used in this gemstone series, the word shebu was rendered "agate" in Exodus 28:19. The prophets Isaiah and Ezekiel mention them, but the word they use is derived from kadkod (Strong's #H3539), which means a "sparkling gem" or what is produced when striking fire from a metal forge.
And I will make thy windows of agates, and thy gates of carbuncles, and all thy borders of pleasant stones (Isaiah 54:12, King James Bible).
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, King James Bible).
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.
This precious stone takes its 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.
Agates were 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. They were 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 that agates were 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!