sedimentary
Banded Chert / Jasper Cognation
Microcrystalline Quartz (SiO2) with Hematite/Limonite
Hardness: 6.5-7 (Mohs scale); Color: Earthy browns, ochre, and dark grey-blue bands; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture)
- Hardness
- 6
- Color
- Earthy browns, ochre, and dark grey-blue bands
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5-7 (Mohs scale); Color: Earthy browns, ochre, and dark grey-blue bands; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture)
Formation & geological history
Formed by the precipitation of silica from water or within sedimentary deposits. The banding occurs due to fluctuations in mineral impurities (like iron oxides) during the long-term deposition of silica-rich fluids.
Uses & applications
Used historically for stone tools due to sharp edges when fractured; modernly used as decorative landscaping stones, polishing for jewelry, or as collector specimens.
Geological facts
This specimen shows 'primary banding' where different mineral concentrations settled. Chert is so durable that it often survives as rounded river pebbles while softer surrounding rocks erode away.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and smooth, glass-like fracture surfaces. Often found in riverbeds, glacial tills, or weathered out of limestone formations.
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