sedimentary
Chert (specifically Flint or Dark Chert)
Silicon Dioxide (SiO2) - Microcrystalline Quartz
Hardness: 7 Mohs scale. Color: Black, dark gray, with tan/cream cortex. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.65.
- Hardness
- 7 Mohs scale
- Color
- Black, dark gray, with tan/cream cortex
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 Mohs scale. Color: Black, dark gray, with tan/cream cortex. Luster: Waxy to dull. Crystal structure: Cryptocrystalline. Cleavage: None (conchoidal fracture). Specific gravity: 2.65.
Formation & geological history
Formed by the precipitation of silica from groundwater or the accumulation of silica-rich organic remains (like radiolarians or diatoms) within sedimentary layers, typically limestone or chalk. Can be millions of years old, often dating from the Paleozoic or Mesozoic eras.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to sharp edges; used in fire-starting (flint and steel); modern uses include abrasives and road surfacing.
Geological facts
This specimen appears to be a 'flake' or debitage, possibly resulting from prehistoric tool-making (knapping). The characteristic 'cortex' (chalky outer layer) is visible at the top.
Field identification & locations
Identify in the field by its shell-like (conchoidal) fracture pattern and its ability to scratch glass. Commonly found in limestone outcrops or as weathered nodules in riverbeds.
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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