mineral

Chalcedony (Agate)

Silicon Dioxide (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Waxy white, light gray, or bluish; Luster: Vitreous to waxy; Crystal structure: Cryptocrystalline (trigonal/monoclinic); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.64.

Hardness: 6
Color: Waxy white, light gray, or bluish
Luster: Vitreous to waxy

Identified March 27, 2026More mineral →

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Physical properties

Formation & geological history

Formed through the deposition of silica from groundwater in volcanic rock cavities or sedimentary environments. It is often secondary in nature, filling vesicles or cracks in older rock formations across various geological ages from Precambrian to Cenozoic.

Uses & applications

Used extensively in jewelry as gemstones, cabochons, and beads. Historically used for tool making (scrapers/arrowheads). In modern industry, it is used for precision bearings and pivot points in instruments due to its hardness.

Geological facts

Agate is a banded variety of chalcedony. The name 'chalcedony' comes from the ancient Greek town of Chalcedon. Because it is cryptocrystalline, its micro-crystals are too small to be seen even under a standard microscope.

Field identification & locations

Identify in the field by its waxy luster, translucency, and inability to be scratched by a steel knife. It often feels 'smoother' than quartz. Commonly found in riverbeds, lake shores (like Lake Superior), and volcanic regions like Brazil, India, and the western USA.