sedimentary
Chert (and/or River Pebble)
Microcrystalline quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Grayish-blue or dark gray; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Grayish-blue or dark gray
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Grayish-blue or dark gray; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation in marine or freshwater environments. The rounded shape indicates it was smoothed by water erosion over a long period.
Uses & applications
Historically used for stone tools and starting fires (flint). Currently used in construction aggregate, landscaping, and sometimes as a decorative river stone.
Geological facts
Chert is so hard it can scratch glass. It was one of the first minerals used by early humans to create sharp cutting tools due to its predictable breaking pattern (conchoidal fracture).
Field identification & locations
Identify by its extreme hardness, waxy texture, and rounded water-worn shape. Often found in riverbeds, glacial deposits, or beach gravels worldwide.
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