sedimentary
Chert (Basanite / Flint variety)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Black to dark gray; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Black to dark gray
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Black to dark gray; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within sedimentary host rocks. Common in limestone and chalk beds.
Uses & applications
Historically used for stone tools and weapons due to sharp edges; currently used for road ballast, friction fire-starting, and as decorative river stones.
Geological facts
Dark black chert is often referred to as 'Basanite' or 'Touchstone' and was historically used by jewelers to test the purity of gold by observing the color of the streak left on the stone.
Field identification & locations
Identify by its 'glassy' smooth texture and curved, shell-like (conchoidal) fractures when broken. Commonly found in riverbeds and coastal areas as rounded pebbles.
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