sedimentary
Chert (specifically River Stone / Jasper variety)
Microcrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Brown, tan, and earthy tones with waxy luster; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 6
- Color
- Brown, tan, and earthy tones with waxy luster
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Brown, tan, and earthy tones with waxy luster; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation from hydrothermal fluids. This specimen appears water-worn, indicating it was shaped by river or stream transport over thousands of years.
Uses & applications
Historically used for stone tools (arrowheads) due to sharp edges; currently used as landscaping gravel, decorative stones, and low-cost lapidary material for tumbling.
Geological facts
Chert is so durable that it can survive long-distance transport in rivers and along beaches where other rocks break down. In antiquity, it was prized alongside flint for its ability to create sparks when struck against steel.
Field identification & locations
Identify in the field by its hardness (it should scratch glass), waxy feel, and smooth curves where it has chipped (conchoidal fracture). It is ubiquitous in riverbeds and gravel pits worldwide.
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Cone-in-cone structure (Secondary Sedimentary Structure)
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Clastic Sedimentary Rock Grain (SiO2 dominated)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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Shale or Slaty Mudstone
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Sandstone
Arenite (primarily SiO2)
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