sedimentary

Chert (and Chert Geode)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: Gray, tan, white; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline/microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Gray, tan, white
Luster: Waxy to vitreous

Identified April 12, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale); Color: Gray, tan, white; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline/microcrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed from the chemical precipitation of silica-rich fluids or the accumulation of siliceous remains (like radiolarians/diatoms) in marine or groundwater environments. Often found as nodules within limestone or chalk beds.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) due to sharp edges; currently used for road gravel, construction aggregate, and occasionally as lapidary material for hobbyists.

Geological facts

Chert is chemically nearly identical to quartz but has a much smaller grain size. A variety called Flint was essential to early human survival for both tools and starting fires.

Field identification & locations

Identify by its waxy appearance and smooth, curved (conchoidal) fractures that are sharp to the touch. This specimen is unique as it contains a small vug or cavity lined with tiny quartz crystals (a geode). Found worldwide, especially in limestone-rich regions.