sedimentary
Chert (Pebble)
Microcrystalline or cryptocrystalline Quartz (SiO2)
Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, or light green; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 6
- Color
- Gray, tan, or light green
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Gray, tan, or light green; Luster: Waxy to dull; 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 radiolarians or diatoms) or by chemical precipitation from groundwater within sedimentary layers. This specific specimen has been water-worn into a smooth pebble shape.
Uses & applications
Historically used for stone tools and weapons (flint-knapping); currently used in road construction, landscaping, and occasionally as lapidary material for tumbling.
Geological facts
Chert is so hard it can scratch glass. It was one of the most important materials for prehistoric humans because it fractures into extremely sharp edges. This variety often shows internal 'ghost' patterns of former silica-rich structures.
Field identification & locations
Look for a smooth, waxy texture and conchoidal (shell-like) fractures if broken. It is a common find on gravel beaches, riverbeds, and within limestone formations worldwide.
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