sedimentary
Chert (Jasper/Flint variety)
Cryptocrystalline Quartz (SiO2)
Hardness: 6.5-7 Mohs; Color: Brown, tan, reddish-orange due to iron oxides; Luster: Dull to waxy; Structure: Cryptocrystalline (no visible crystals); Cleavage: None (conchoidal fracture).
- Hardness
- 6
- Color
- Brown, tan, reddish-orange due to iron oxides
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5-7 Mohs; Color: Brown, tan, reddish-orange due to iron oxides; Luster: Dull to waxy; Structure: Cryptocrystalline (no visible crystals); Cleavage: None (conchoidal fracture).
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by the chemical replacement of limestone/dolomite by silica-rich groundwater. Often found in nodules or bedded deposits.
Uses & applications
Historically used for stone tools/weapons due to conchoidal fracture. Modernly used in road surfacing, landscaping, and lapidary work for cabochons.
Geological facts
Chert is so hard it can scratch steel. When struck against steel, it can produce sparks, which is why the 'flint' variety was historically significant for starting fires and in flintlock firearms.
Field identification & locations
Identify by its smooth, waxy feel and distinct curved (conchoidal) fractures where it has chipped. Commonly found in riverbeds, glacial tills, and limestone outcrops.
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