sedimentary
Chert (Siliceous Mudstone variant)
Microcrystalline quartz (SiO2)
Hardness: 6.5-7 (Mohs). Color: Dark gray to brown with earthy patinas. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.65.
- Hardness
- 6
- Color
- Dark gray to brown with earthy patinas
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs). Color: Dark gray to brown with earthy patinas. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific Gravity: 2.65.
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation in marine environments. Often found as nodules within limestone or as bedded deposits.
Uses & applications
Historically used for stone tools (knapping). Currently used as road aggregate, in grinding media, and occasionally as a gemstone when colorful (Jasper).
Geological facts
Chert is so hard it can scratch steel and was once used in flintlock firearms to create sparks. It is biologically derived, essentially representing ancient sea floor accumulation.
Field identification & locations
Identify in the field by its hardness and sharp, glass-like edges when broken. Frequently found in riverbeds or eroding out of limestone cliffs. Often looks like an ordinary dull rock until fractured.
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