sedimentary
Chert (Tan/Grey variety)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, beige, or light grey; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Tan, beige, or light grey
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, beige, or light grey; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of siliceous remains of organisms (like diatoms or radiolarians) or chemical precipitation from silica-rich fluids in sedimentary basins. Often occurs as nodules in carbonate rocks.
Uses & applications
Historically used for stone tools due to sharp edges; currently used as road aggregate, in landscaping, and as a polishing media in rock tumblers.
Geological facts
Chert was a primary material for prehistoric toolmaking globally; when struck against steel, it can produce sparks, though less effectively than flint.
Field identification & locations
Identify by its characteristic conchoidal (shell-like) fracture patterns and ability to scratch glass. Commonly found in limestone and dolomite formations or as river pebbles.
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