sedimentary
Chert (Smoky Variety)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Brown, grey, tan; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Brown, grey, tan
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Brown, grey, tan; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich biogenic remains (like radiolaria or diatoms) or chemical precipitation from hydrothermal fluids in sedimentary basins. Most common in Paleozoic and Mesozoic strata.
Uses & applications
Historically used for stone tools and weapons due to sharp brittle edges; currently used as road aggregate, architectural stone, and occasionally in lapidary work for cabochons.
Geological facts
Chert is so durable that it often remains as pebbles in stream beds long after the surrounding limestone or shale has weathered away. It was one of the most important minerals for early human technological development.
Field identification & locations
Identify by its extreme hardness (scratches glass), waxy luster on fresh surfaces, and characteristic curved (conchoidal) breaking pattern. Found worldwide in nodules or bedded layers within limestone.
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