sedimentary
Banded Chert (Flint)
Cryptocrystalline Quartz (SiO2)
Hardness: 7 Mohs scale; Color: Dark brown, amber, and grey banding; Luster: Waxy to vitreous; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 Mohs scale
- Color
- Dark brown, amber, and grey banding
- Luster
- Waxy to vitreous
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Physical properties
Hardness: 7 Mohs scale; Color: Dark brown, amber, and grey banding; Luster: Waxy to vitreous; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed in marine environments through the precipitation of silica from groundwater or the accumulation of silica-rich organic remains (like radiolarians or diatoms) during the Paleozoic to Mesozoic eras.
Uses & applications
Historically used for stone tools and fire-starting; currently used as ornamental stones, in jewelry (cabs), and as a silica source in industrial glass manufacturing.
Geological facts
The conchoidal fracture creates extremely sharp edges, making it the primary material for prehistoric arrowheads and knives. High-quality banded versions are often called 'Chert Onyx' in the lapidary trade.
Field identification & locations
Identified by its extreme hardness (scratches glass), waxy appearance, and shell-like (conchoidal) breaks. Often found in limestone or chalk beds globally, specifically in the UK, USA, and France.
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