sedimentary

Chert

Cryptocrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: light grey to off-white; Luster: waxy to dull; Crystal structure: cryptocrystalline (microscopic); Cleavage: none, exhibits conchoidal fracture.

Hardness: 7 on Mohs scale
Color: light grey to off-white
Luster: waxy to dull

Identified June 1, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: light grey to off-white; Luster: waxy to dull; Crystal structure: cryptocrystalline (microscopic); Cleavage: none, exhibits conchoidal fracture.

Formation & geological history

Formed by the accumulation of siliceous remains of marine organisms (like diatoms or radiolarians) or through the chemical precipitation of silica in groundwater. Found in sedimentary bedding or as nodules in limestone.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges. Currently used in road construction, as an abrasive, and sometimes in jewelry for tumbled stones.

Geological facts

Chert is so hard it can create sparks when struck against steel, which made it vital for flintlock firearms. Flint is a specific variety of dark chert.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) breaking pattern. Frequently found in limestone quarries or riverbeds.