sedimentary
Chert (Flint)
Chert (Cryptocrystalline Quartz, SiO2)
Hardness: 7 on Mohs scale; Color: Grey, black, or brown; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Grey, black, or brown
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Grey, black, or brown; Luster: Dull to waxy; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Forms as nodules or layers within sedimentary rocks like limestone or chalk. It is composed of the remains of silica-rich organisms or chemical precipitates. Common across many geological ages, particularly Paleozoic and Mesozoic.
Uses & applications
Historically used for stone tools (knapping), arrowheads, and fire-starting. Currently used as road aggregate and as an abrasive in some industrial processes. High-quality colorful pieces are collected by rockhounds.
Geological facts
Chert has a conchoidal fracture which allows it to produce razor-sharp edges. It was one of the most important minerals for early human survival for both hunting and ignition.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by steel) and curved, glass-like fracture surfaces. Often found in riverbeds or as nodules inside limestone cliffs. Look for smooth, waxy surfaces on fresh breaks.
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