sedimentary
Chert (Iron-stained)
Chert (SiO2)
Hardness: 7 (Mohs scale); Color: Dark brown to grey with rust-toned oxidation; Luster: Dull to waxy; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 7 (Mohs scale)
- Color
- Dark brown to grey with rust-toned oxidation
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Dark brown to grey with rust-toned oxidation; Luster: Dull to waxy; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation in marine or freshwater environments. The dark/rusty color suggests iron oxide infiltration during or after deposition.
Uses & applications
Historically used for stone tools (flintknapping) due to its sharp edges; modernly used as road aggregate, architectural stone, and for lapidary tumbling.
Geological facts
Chert is so durable that it often survives as rounded pebbles in stream beds long after the surrounding rock has eroded away. It is chemically identical to quartz but has a much finer grain structure.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in limestone and chalk beds or as river cobbles.
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