sedimentary
Banded Chert
Sedimentary Cryptocrystalline Quartz (SiO2)
Hardness: 6.5-7 (Mohs), Color: Dark brown to black with lighter tan bands, Luster: Waxy to vitreous when wet, Structure: Cryptocrystalline, Fracture: Conchoidal, Cleavage: None
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs), Color: Dark brown to black with lighter tan bands, Luster: Waxy to vitreous when wet, Structure: Cryptocrystalline, Fracture: Conchoidal, Cleavage: None
Formation & geological history
Formed through the precipitation of silica-rich fluids in marine environments or the accumulation of microcrystalline quartz from radiolarians and diatoms. Often related to Proterozoic or Paleozoic formations.
Uses & applications
Historically used for stone tools/arrowheads. Modernly used for aquarium decor, lapidary tumbling, and as garden gravel.
Geological facts
The distinct banding is caused by rhythmic deposition of iron oxides or organic matter during the formation of the silica layers. It is highly resistant to chemical weathering.
Field identification & locations
Commonly found in riverbeds, glacial till, and coastal areas as smooth 'water-worn' pebbles. Identify by its high hardness (scratches glass) and smooth, shell-like fracture pattern.
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