sedimentary
Chert (Flint Nodules)
Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: typically grey, tan, or brown; Luster: dull to waxy; Texture: fine-grained/cryptocrystalline; Fractures: conchoidal (producing sharp edges).
- Hardness
- 7 on Mohs scale
- Color
- typically grey, tan, or brown
- Luster
- dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: typically grey, tan, or brown; Luster: dull to waxy; Texture: fine-grained/cryptocrystalline; Fractures: conchoidal (producing sharp edges).
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation within limestone or chalk beds. Often found as nodules or concretions in marine sedimentary environments.
Uses & applications
Historically used for stone tools and starting fires; currently used in construction aggregates, road surfacing, and as a decorative stone in landscaping.
Geological facts
Because it breaks with a conchoidal fracture like glass, it was the primary material for prehistoric arrowheads and scrapers. It is chemically very resistant to weathering.
Field identification & locations
Identify by its hardness (will scratch glass), conchoidal fracture patterns, and smooth but dull appearance. Commonly found along riverbeds, beaches, and eroded limestone outcroppings.
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