sedimentary
Chert (River Pebble)
Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: Tan to yellowish-brown; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Tan to yellowish-brown
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan to yellowish-brown; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed via the accumulation of silica-rich organic remains or chemical precipitation in marine or riverine environments. Often occurs as nodules within limestone or as secondary deposits in stream beds.
Uses & applications
Historically used for stone tools and fire-starting; currently used in construction aggregate, landscaping, and lapidary for tumbled stones.
Geological facts
Chert has been used by humans for millions of years to create sharp-edged tools due to its conchoidal fracturing. This specific specimen shows 'desert varnish' or water-worn smoothing from transport in a river.
Field identification & locations
Identify in the field by its hardness (cannot be scratched by a steel knife) and its smooth, waxy surface. Common in gravel pits and riverbanks globally.
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Cone-in-cone structure (Limestone)
Cone-in-cone structure (Secondary Sedimentary Structure)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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Clastic Sedimentary Rock Grain (SiO2 dominated)
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Shale or Slaty Mudstone
Argillaceous sedimentary rock
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Sandstone
Arenite (primarily SiO2)
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