sedimentary
Chert (River Pebble)
Microcrystalline Silica (SiO2)
Hardness: 6.5-7 Mohs; Color: Tan, brown, or ochre; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6
- Hardness
- 6
- Color
- Tan, brown, or ochre
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 Mohs; Color: Tan, brown, or ochre; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation in marine environments, later rounded and smoothed by water transport in a river or stream.
Uses & applications
Historical use for tool making (flint knapping), modern use as aggregate, decorative landscaping stone, or tumbled for collection.
Geological facts
Chert is chemically the same as quartz but lacks visible crystals. Because it breaks with a sharp edge, it was one of the primary materials used by prehistoric humans to create arrowheads and scrapers.
Field identification & locations
Identify in the field by its waxy texture, smooth water-worn surface, and ability to scratch glass. It often shows 'crescent' impact marks from hitting other rocks in rivers.
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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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