mineral

Chalcedony (Agate / Chert)

Silicon Dioxide (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Greyish-blue to translucent light blue/grey; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline quartz); Cleavage: None; Conchoidal fracture

Hardness: 6
Color: Greyish-blue to translucent light blue/grey
Luster: Waxy to vitreous

Identified July 24, 2026More mineral →

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Physical properties

Hardness: 6.5-7 on Mohs scale; Color: Greyish-blue to translucent light blue/grey; Luster: Waxy to vitreous; Crystal structure: Trigonal (microcrystalline quartz); Cleavage: None; Conchoidal fracture

Formation & geological history

Formed by the deposition of silica-rich fluids at relatively low temperatures in cavities and fractures of various rock types, including volcanic and sedimentary rocks.

Uses & applications

Used in lapidary arts, jewelry making, tumbling, ornamental carvings, and historical tool making (flint knapping).

Geological facts

Chalcedony is a cryptocrystalline form of silica, composed of very fine intergrowths of quartz and moganite. Agate is a banded variety of chalcedony.

Field identification & locations

Identified by its waxy luster, smooth conchoidal (shell-like) fracture surfaces, and hardness (scratches glass). Commonly found in gravel beds, volcanic areas, and dry riverbeds worldwide.