sedimentary

Chert (Flinty variety)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale); Color: Light gray to tan with brownish-orange iron staining; Luster: Waxy to dull; Crystal structure: Hexagonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Light gray to tan with brownish-orange iron staining
Luster: Waxy to dull

Identified February 21, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed primarily through the chemical precipitation of silica-rich fluids within groundwater or the accumulation of siliceous skeletal remains of marine organisms (like diatoms or radiolarians). It can form as nodules in limestone or as bedded deposits.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges; currently used as construction aggregate, road surfacing, and occasionally for lapidary work or fire-starting kits.

Geological facts

Chert has been used by humans for over 2 million years. Its ability to produce sparks when struck against steel made it essential for flintlock firearms and early fire-making.

Field identification & locations

Identify in the field by its very smooth, glass-like curved (conchoidal) surface when broken and its ability to scratch glass. Commonly found in limestone outcrops, riverbeds, and coastal areas.

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Brecciated Chert (Microcrystalline Silica)

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