sedimentary
Agate
Silicon Dioxide (Chalcedony), SiO2
Hardness of 6.5 to 7 on the Mohs scale, variable colors including white, grey, blue, yellow, and brown, waxy to vitreous luster, cryptocrystalline crystal structure, conchoidal fracture with no true cleavage, and specific gravity around 2.60 to 2.65.
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Physical properties
Hardness of 6.5 to 7 on the Mohs scale, variable colors including white, grey, blue, yellow, and brown, waxy to vitreous luster, cryptocrystalline crystal structure, conchoidal fracture with no true cleavage, and specific gravity around 2.60 to 2.65.
Formation & geological history
Formed mainly by the deposition of silica from groundwater in the cavities of igneous, metamorphic, or sedimentary rocks, often developing in concentric bands over millions of years.
Uses & applications
Used in ornamental objects, jewelry, polished specimens, precision pendulum supports, and as a popular collectible mineral.
Geological facts
Agate was named by the Greek philosopher Theophrastus after the Achates river in Sicily, where the stones were found in ancient times. It has been used for talismans and ornamentation since Neolithic times.
Field identification & locations
Can be identified by its distinctive waxy luster, microcrystalline bands or cavities, and high hardness that scratches glass. Commonly found worldwide in volcanic rock cavities, particularly in Brazil, Uruguay, India, and the USA.
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