sedimentary

Chert (Field Pebble)

Microcrystalline / Cryptocrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Hardness: 7 on Mohs scale; Color: Brown, tan, and grey with white specks; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Brown, tan, and grey with white specks
Luster: Dull to waxy

Identified May 3, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Brown, tan, and grey with white specks; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Formation & geological history

Formed through the precipitation of silica from groundwater or the accumulation of silica-rich organic remains (like radiolarians or diatoms) on the seafloor. This specimen shows significant rounding from fluvial (river) or beach transport.

Uses & applications

Historically used for making stone tools (arrowheads/scrapers) due to its sharp edges; modernly used as road aggregate, in landscaping, or as garden pebbles.

Geological facts

Chert is so hard it can strike sparks against steel, making it a primary material for flintlock firearms and primitive fire-starting. The white specks are often tiny fossil fragments or concentrations of purer silica.

Field identification & locations

Identify by its extreme hardness (cannot be scratched by a steel nail), waxy texture when wet, and smooth, curved (conchoidal) fractures. Found worldwide in riverbeds and gravel pits.