sedimentary
Chert (Common Opal/Quartz mix)
Microcrystalline Silicon Dioxide (SiO2)
Hardness: 6.5-7 (Mohs scale), Color: tan, cream, and waxy yellow, Luster: waxy to dull, Cleavage: none (conchoidal fracture), Specific Gravity: 2.6
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs scale), Color: tan, cream, and waxy yellow, Luster: waxy to dull, Cleavage: none (conchoidal fracture), Specific Gravity: 2.6
Formation & geological history
Formed by the precipitation of silica-rich fluids in limestone or chalk beds, or from the accumulation of siliceous remains of diatoms/radiolarians. Often found in nodules within sedimentary layers.
Uses & applications
Historically used for stone tools and arrowheads due to sharp edges; currently used for lapidary work, decorative landscaping, and occasionally as a source of industrial silica.
Geological facts
Chert has been used by humans for millions of years to make fire and tools because it breaks with a conchoidal fracture, creating razor-sharp edges. Flint is a dark variety of chert.
Field identification & locations
Identify in the field by its waxy appearance, extreme hardness (scratches glass), and the way it breaks into curved, shell-like flakes. Found globally in sedimentary environments.
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