sedimentary
Chert (Polished/Tumbled)
Silicon Dioxide (SiO2)
Hardness: 7 Mohs; Color: Black with tan/brown bands; Luster: Waxy to vitreous; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); SG: 2.65
- Hardness
- 7 Mohs
- Color
- Black with tan/brown bands
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 Mohs; Color: Black with tan/brown bands; Luster: Waxy to vitreous; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); SG: 2.65
Formation & geological history
Formed as a chemical precipitate or biochemical deposit in marine environments, where silica shells of organisms or dissolved silica accumulate and harden into rock over millions of years.
Uses & applications
Used for lapidary work (polishing, tumbling), jewelry accents, and historically for stone tools due to its sharp conchoidal fracture.
Geological facts
Chert that is darkly colored is often referred to as flint. This specific specimen shows characteristic bands of different mineral impurities, likely iron or organic matter, and has been tumbled to a smooth finish.
Field identification & locations
Identify in the field by its hardness (scratches glass), waxy feel, and smooth curved fractures. Commonly found in limestone or chalk beds and along riverbanks/beaches as rounded pebbles.
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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