sedimentary
Chert (and Limestone Pebbles)
Silicon Dioxide (SiO2) / Sedimentary Microcrystalline Quartz
Hardness: 6.5-7 Mohs; Color: White, tan, brown, grey; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
- Hardness
- 6
- Color
- White, tan, brown, grey
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 Mohs; Color: White, tan, brown, grey; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms) or chemical precipitation from groundwater within limestone/chalk beds. These specimens are fluvially worn river stones.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to conchoidal fracturing. Modernly used in construction aggregate, landscaping, and flint-making for fire starting.
Geological facts
Chert is so durable that it often remains as pebbles in riverbeds long after the surrounding limestone host rock has eroded away. The specimen in the center shows 'vugs' or pits where softer minerals have weathered out.
Field identification & locations
Identify by its hardness (scratches glass), waxy appearance, and smooth rounded shape in river deposits. Common globally in areas with ancient seafloor sedimentary deposits.
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