sedimentary
Chert (variations include Flint/Jasper)
Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale). Color: Highly variable (grey, brown, tan). Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.6 – 2.7.
- Hardness
- 7 (Mohs scale)
- Color
- Highly variable (grey, brown, tan)
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale). Color: Highly variable (grey, brown, tan). Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.6 – 2.7.
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like radiolarians or diatoms) or through chemical precipitation in limestone/chalk beds. They appear as nodules or layered beds in sedimentary formations ranging from Precambrian to Cenozoic ages.
Uses & applications
Historically used for stone tools and weapons due to sharp edges; currently used for road surfacing, grinding media, and occasionally as semi-precious gemstones in jewelry (as Jasper).
Geological facts
Chert was one of the most important minerals for early humans because of its predictable conchoidal fracture patterns, allowing it to be 'knapped' into arrowheads and scrapers.
Field identification & locations
Identify in the field by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in limestone outcrops or as rounded pebbles in riverbeds.
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