sedimentary
Chert
Chert (Microcrystalline Quartz, SiO2)
Hardness: 7 Mohs scale; Color: Dark grey to black with tan rinds; Luster: Waxy to dull; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 Mohs scale
- Color
- Dark grey to black with tan rinds
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 Mohs scale; Color: Dark grey to black with tan rinds; Luster: Waxy to dull; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed via the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation on the seafloor, later undergoing lithification over millions of years. This specimen shows a weathered outer rind or patina.
Uses & applications
Historically used for stone tools and arrowheads due to its sharp edges; modernly used as road aggregate, architectural stone, and for fire-starting (flint).
Geological facts
Chert is essentially the same mineral as flint, though 'flint' usually refers to chert found in chalk or marly limestone. It produces sparks when struck against steel.
Field identification & locations
Identify by its extreme hardness (scratches glass) and its curved, shell-like (conchoidal) fracture surfaces seen on the broken face. Common in limestone riverbeds.
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