Agate was named by Theophrastus after he discovered the stone along the shore of the Archates in the 4th century BCE, but was in use long before then, seen in hard stone carvings as much as 3,000 years old. It's a crysptocrystalline silica formed in the cavities of volcanic rock. (Cryptocrystalline means that the crystals are so very small that their structure can only be seen in a very thin slice, under a microscope, and with a specific light.) Its hardness and chemical resistance make it appropriate for industrial use and even mortars and pestles, but its innate beauty has had it used in jewellery, ornamentation, and art.
According to The Book of Talismans, Amulets, and Zodiacal Gems by William Thomas and Kate Pavitt, agate is beneficial to the sign of Gemini, and Greeks and Romans wore it as a talisman and believed that it could help cure the poison of an asp or scorpion. It was also believed that burning the stone could avert storms and wearing them could increase strength, success, and vigor. Additionally, it has been believed to prevent fever, bring luck in the form of inheritance, bring enlightenment, and make the wearer more amiable.
Agate also appears in Exodus 28 & 39 as a stone on the breastplate of a priest. According to Philippine Folklore Stories by Mabel Cook Cole, agate beads are highly valued by Tinguian women and are rarely sold.