sedimentary
Chert (containing small fossils)
Chert (Microcrystalline Quartz, SiO2)
Hardness: 7 (Mohs scale); Color: Light tan/gray with darker inclusions; Luster: Dull to waxy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Light tan/gray with darker inclusions
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Light tan/gray with darker inclusions; Luster: Dull to waxy; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation in marine environments. Often found in Phanerozoic limestone beds (541 million years ago to present).
Uses & applications
Historically used for making sharp tools and arrowheads. Used today in road construction, as an abrasive, and occasionally for lapidary work if highly pigmented.
Geological facts
Chert is so hard it can create sparks when struck against steel. Evidence of ancient marine life is often preserved within its dense structure because the silica replaces organic material perfectly.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel blade) and smooth, shell-like (conchoidal) fractures. Found frequently in riverbeds or eroding out of limestone cliffs. Look for small, dark specks which may be fossil fragments.
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