sedimentary
Chert (Smooth River Pebble)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Tan, brown, or grayish; Luster: Waxy to dull (though this specimen appears smooth/polished); Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 6
- Color
- Tan, brown, or grayish
- Luster
- Waxy to dull (though this specimen appears smooth/polished)
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Tan, brown, or grayish; Luster: Waxy to dull (though this specimen appears smooth/polished); Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation from groundwater within limestone or chalk beds. This specific specimen has been rounded and smoothed by water erosion in a high-energy environment like a river or beach.
Uses & applications
Historically used for making stone tools (arrowheads and scrapers) due to its conchoidal fracture. Modernly used in landscaping, as road aggregate, and as 'river rocks' for decorative stone mulch.
Geological facts
Chert is remarkably durable and resistant to chemical weathering. When struck against steel, it can produce sparks, which led to its historical use in flintlock firearms (specifically the variety known as flint).
Field identification & locations
Identify in the field by its hardness (it will scratch glass), waxy texture, and smooth, rounded water-worn shape. It is one of the most common pebbles found in gravel bars and riverbeds globally.
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