sedimentary
Chert (containing Fossil Inclusions)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, tan, or black with lighter inclusions; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, tan, or black with lighter inclusions
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, tan, or black with lighter inclusions; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed from the precipitation of silica-rich fluids in limestone or chalk beds, often replacing organic material. These specimens typically range from the Paleozoic to Mesozoic eras (approx. 66 to 500 million years old).
Uses & applications
Historically used for stone tools (flintknapping) due to its sharp edges. Modernly used as road aggregate, architectural stone, and for educational fossil collecting.
Geological facts
Chert nodules often trap ancient marine life like crinoids or brachiopods. Because silica is so durable, these fossils remain intact long after the surrounding limestone has eroded away.
Field identification & locations
Identify by its 'shell-like' conchoidal fracture and its ability to scratch glass. Commonly found in riverbeds or gravel pits in limestone regions like the American Midwest or the UK.
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