sedimentary
Chert (containing fossil leaf impression)
Microcrystalline silicon dioxide (SiO2)
Hardness: 7 (Mohs scale); Color: Light gray to sage green; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Light gray to sage green
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Light gray to sage green; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the precipitation of silica from groundwater within sedimentary layers, often replacing limestone or shale. The specimen displays a carbon film or impression of a leaf, likely from the Cenozoic era, preserved in fine-grained sediment that later silicified.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to its sharp conchoidal fracture. Modernly used in construction aggregate and as a collector's specimen for fossil enthusiasts.
Geological facts
Chert is so durable that it can survive long periods of weathering that destroy surrounding rocks. The fossilized leaf impression indicates a terrestrial environment nearby during the time of deposition, capturing a 'snapshot' of ancient flora.
Field identification & locations
Identify in the field by its high hardness (can scratch glass) and smooth, shell-like fracture surfaces. Often found in riverbeds or limestone outcrops. Collectors should look for fine-patterned leaf veins to verify biological origin.
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