sedimentary
Chert (Field Pebble)
Microcrystalline / Cryptocrystalline Quartz (SiO2)
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
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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.
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