sedimentary
Chert (and Jasper/Flint variety)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Gray, tan, brown, with iron staining; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Gray, tan, brown, with iron staining
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, tan, brown, with iron staining; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the precipitation of silica from seawater or the replacement of limestone/dolomite by silica. This specific sample shows significant weathering and possible hydrothermal alteration, likely from the Paleozoic or Mesozoic era.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to conchoidal fracture. Modern uses include road aggregate, decorative landscaping, and as a source of silica for glassmaking.
Geological facts
Chert is so hard it can spark when struck against steel. It is frequently found as nodules within limestone or as massive bedded deposits in deep-sea environments.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) break patterns. Look for it in sedimentary basins or as rounded river cobbles.
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