sedimentary

Chert

Microcrystalline Silicon Dioxide (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: White to tan/light gray; Luster: Waxy to dull/earthy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6

Hardness: 6
Color: White to tan/light gray
Luster: Waxy to dull/earthy

Identified May 31, 2026More sedimentary →

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Physical properties

Hardness: 6.5-7 on Mohs scale; Color: White to tan/light gray; Luster: Waxy to dull/earthy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6

Formation & geological history

Formed in sedimentary environments from the accumulation of silica-rich organic remains (like diatoms or radiolaria) or through chemical precipitation in groundwater replacing limestone. Found in strata ranging from Precambrian to recent.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; used today as a construction aggregate and in road surfacing.

Geological facts

This specimen shows evidence of a conchoidal fracture pattern, which made it a primary material for prehistoric humans to create 'lithic' tools. When struck with steel, chert can produce sparks.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like fracture surfaces. Frequently found as nodules within limestone or as bedded layers in marine sedimentary rocks.