sedimentary
Chert (Iron-rich or Slate-like variant)
Silicon Dioxide (SiO2)
Hardness: 7 Mohs; Color: Dark grey to charcoal black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz crystals); Cleavage: None/Conchoidal fracture; SG: 2.65
- Hardness
- 7 Mohs
- Color
- Dark grey to charcoal black
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 Mohs; Color: Dark grey to charcoal black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz crystals); Cleavage: None/Conchoidal fracture; SG: 2.65
Formation & geological history
Formed by the accumulation of siliceous remains of marine organisms or chemical precipitation from hydrothermal fluids in sedimentary basins. Often found in nodules or bedded layers.
Uses & applications
Historically used for stone tools (knapping) due to its sharp edges. Modernly used in construction aggregate, road base, and as decorative gravel.
Geological facts
The black color in this specimen is likely due to high organic carbon content or inclusions of manganese/iron oxides. Chert is so hard it can strike sparks from steel, which led to its historical use as 'flint'.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its curved, glass-like (conchoidal) fracture edges. Commonly found in riverbeds or limestone outcrops.
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