sedimentary
Agate
Silicon dioxide (Chalcedony), SiO2
Hardness of 6.5 to 7 on the Mohs scale, waxy to vitreous luster, microcrystalline quartz crystal structure, conchoidal fracture with no true cleavage, and specific gravity of 2.60 to 2.65. Colors vary widely due to impurities including white, gray, blue, red, orange, and brown.
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Physical properties
Hardness of 6.5 to 7 on the Mohs scale, waxy to vitreous luster, microcrystalline quartz crystal structure, conchoidal fracture with no true cleavage, and specific gravity of 2.60 to 2.65. Colors vary widely due to impurities including white, gray, blue, red, orange, and brown.
Formation & geological history
Formed primarily within cavities of volcanic and metamorphic rocks through the deposition of silica-rich groundwater solutions in concentric layers over long periods.
Uses & applications
Used in ornamental objects, jewelry, lapidary art, and occasionally for precision instruments due to its chemical resistance and hardness.
Geological facts
Agate has been used since antiquity for gemstones, seals, and talismans. The name comes from the Dirilo River (now Achates) in Sicily, where agates were found in ancient times.
Field identification & locations
Easily identified by its distinctive banded patterns, waxy luster, and conchoidal fracture. Often found in gravel beds, beaches, and weathered volcanic rock formations worldwide.
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