sedimentary
Chert with Calcite Veins
Silicon Dioxide (SiO2) with Calcium Carbonate (CaCO3) intrusions
Hardness: 7 (matrix), 3 (veins); Color: Dark grey to black with white streaks; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture)
- Hardness
- 7 (matrix), 3 (veins)
- Color
- Dark grey to black with white streaks
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (matrix), 3 (veins); Color: Dark grey to black with white streaks; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (Conchoidal fracture)
Formation & geological history
Formed from the accumulation of silica-rich organic remains in marine environments. The white veins represent secondary mineralization where mineral-rich water filled fractures in the original rock.
Uses & applications
Used historically for stone tools due to its sharp edges; today used for landscaping, as river stones, and sometimes for decorative lapidary work.
Geological facts
Dark-colored chert is often called Flint. The white veins are typically 'crack-seal' veins, indicating multiple pulses of fluid injection during tectonic activity.
Field identification & locations
Identify by its hardness (the dark part will scratch glass) and white veins that react to dilute acid. Common in riverbeds and coastal areas.
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