sedimentary
Banded Chert
Silicon Dioxide (SiO2)
Hardness: 7 Mohs; Color: Black, blue-grey, tan, and white bands; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 Mohs
- Color
- Black, blue-grey, tan, and white bands
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 Mohs; Color: Black, blue-grey, tan, and white bands; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation in marine environments. Banding represents seasonal or cyclical changes in sediment deposition.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to sharp edges; currently used for jewelry cabochons, lapidary art, and decor.
Geological facts
Banded chert is often associated with some of the oldest known fossils on Earth, such as stromatolites found in Precambrian formations.
Field identification & locations
Identify by its extreme hardness (scratches glass), waxy feel, and smooth curved (conchoidal) fractures. Common in limestone or dolostone beds globally.
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