sedimentary
Chert (Commonly known as Jasper or Flint)
Microcrystalline silica (SiO2)
Hardness: 6.5-7 Mohs; Color: Tan, brown, and grey; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (hexagonal system at microscopic level); Cleavage: None (conchoidal fracture); Specific gravity: 2.6
- Hardness
- 6
- Color
- Tan, brown, and grey
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 Mohs; Color: Tan, brown, and grey; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (hexagonal system at microscopic level); Cleavage: None (conchoidal fracture); Specific gravity: 2.6
Formation & geological history
Formed primarily from the accumulation of silica-rich microfossils like radiolaria or diatoms on the ocean floor, which undergo recrystallization over millions of years. This specimen shows water-worn rounding.
Uses & applications
Historically used for stone tools and fire-starting. Today used in jewelry (as jasper), landscaping, and as a polishing media in rock tumblers.
Geological facts
Chert is so durable that it often survives long after the surrounding rock has eroded, which is why it is frequently found as rounded river or beach pebbles. It has been used by humans for over 2 million years to create sharp-edged tools.
Field identification & locations
Identify by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds, glacial tills, and coastal beaches.
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