sedimentary

Banded Chert / Agate

Silicon Dioxide (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, yellowish-brown, and off-white in distinct bands; Luster: Waxy to vitreous (glassy); Cleavage: None (features conchoidal fracture); Specific Gravity: ~2.65.

Hardness: 6
Color: Gray, tan, yellowish-brown, and off-white in distinct bands
Luster: Waxy to vitreous (glassy)

Identified July 26, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed via the precipitation of microcrystalline silica from water, often replacing organic or carbonate material in sedimentary marine or lacustrine environments, or through secondary silica deposition in volcanic cavities over millions of years.

Uses & applications

Used historically for knapping into cutting tools and arrowheads due to its sharp conchoidal fracture; currently popular as a lapidary material, decorative stone, and collector specimen.

Geological facts

Banded chert and chalcedony can preserve ancient microscopic textures and fossils. The distinct colored banding comes from trace iron oxides and clay inclusions that varied during deposition.

Field identification & locations

Identified by its microcrystalline structure, smooth waxy surface, sharp shell-like (conchoidal) fracture edges, and hardness capable of scratching steel or glass. Commonly found in riverbeds, gravel pits, and sedimentary rock formations worldwide.