sedimentary
Chert Pebble
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 7 on the Mohs scale; Color: Tan to light brown; Luster: Waxy to dull (weathered); Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on the Mohs scale
- Color
- Tan to light brown
- Luster
- Waxy to dull (weathered)
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Physical properties
Hardness: 7 on the Mohs scale; Color: Tan to light brown; Luster: Waxy to dull (weathered); Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within limestone or chalk beds. This specimen is water-worn and likely found in a riverbed or alluvial deposit.
Uses & applications
Historically used for stone tools and starting fires (flint); currently used as a road aggregate, in landscaping, and as a durable abrasive in grit-blasting.
Geological facts
Chert is so hard that it can scratch steel and glass. Because it breaks with a conchoidal (shell-like) fracture, it was the primary material used by prehistoric humans to create sharp arrowheads and knives.
Field identification & locations
Identify in the field by its extreme hardness and smooth, waxy texture when wet. It will not react with acid, unlike the limestone it is often found in. Commonly found in gravel pits, stream beds, and ocean beaches worldwide.
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