sedimentary

Agate

Silicon Dioxide (Chalcedony), SiO2

Hardness of 6.5 to 7 on the Mohs scale, waxy to vitreous luster, cryptocrystalline crystal structure (microscopic quartz crystals), conchoidal fracture, specific gravity of 2.60 to 2.65, translucent to opaque, and often exhibits banded patterns of various colors caused by impurities.

Identified August 1, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed primarily within cavities of volcanic and metamorphic rocks through the slow deposition of silica-rich groundwater solutions over millions of years, often in concentric layers.

Uses & applications

Widely used in jewelry, ornamental carvings, polished specimens for collectors, and historically in precision instruments like mortar and pestles due to its high durability.

Geological facts

Agate was highly prized in the ancient world for use in signet rings and ornamental vessels, and it is named after the Achates River in Sicily where stones were found in antiquity.

Field identification & locations

Can be identified in the field by its waxy luster, micro-banded appearance, conchoidal fracture, and resistance to scratching by a steel knife. Found globally, especially in regions with historic volcanic activity such as Brazil, Uruguay, India, Madagascar, and the US Lake Superior region.