From The Gemology Project
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Chemical composition Silicon dioxide SiO2
Crystal system Trigonal
Habit Aggregate
Fracture Conchoidal
Hardness 6.5-7
Refractive index 1.535 - 1.539
Specific gravity 2.60
Lustre Vitreous

Agate is a variety of banded microcrystalline quartz, or chalcedony. There are countless varieties of agate usually named after their physical characteristics or location of origin. The layers of agate form as layers of chalcedony line a cavity (or vug) in a host rock. Since the 12th century, agate has been attributed with the power of improving one's disposition as well as their eyesight. These beliefs still carry on today. One claim for agate was stated in The Book of Saxon Leechdoms written in 1864, where it was written that agate prevents harm from thunder, sorcery, demonic possession, poison and drunkenness!