sedimentary

Chert (White/Gray variety)

Chert (SiO2)

Hardness: 7 on Mohs scale; Color: typically white, gray, or tan; Luster: waxy to dull; Crystal structure: cryptocrystalline (microscopic quartz); Cleavage: none; Fracture: conchoidal (shell-like); Specific gravity: 2.6

Hardness: 7 on Mohs scale
Color: typically white, gray, or tan
Luster: waxy to dull

Identified March 28, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of siliceous remains of marine organisms (like radiolaria or diatoms) or by chemical precipitation from silica-rich fluids. Often found in nodules within limestone or as bedded layers. Can age from Precambrian to Cenozoic.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to sharp edges; currently used as construction aggregate, road base material, and occasionally in lapidary work as a polishing stone.

Geological facts

Chert is remarkably resistant to chemical weathering and is often the only rock left standing in ancient riverbeds. Flint is a specific dark variety of chert. It can produce sparks when struck against steel.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its curved, sharp conchoidal fracture patterns. Found globally in sedimentary basins. Collectors look for pieces with interesting color banding or fossil inclusions.