sedimentary
Chert (Septarian Nodules-like)
Silicon dioxide (SiO2)
Hardness: 6.5-7 (Mohs scale); Color: Brown to dark grey with lighter infill; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Conchoidal fracture where chipped.
- Hardness
- 6
- Color
- Brown to dark grey with lighter infill
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5-7 (Mohs scale); Color: Brown to dark grey with lighter infill; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Conchoidal fracture where chipped.
Formation & geological history
Formed as a chemical precipitate in sedimentary environments, often replacing organic materials or forming within carbonate rocks. The cracks (septaria) may result from dehydration of a gel-like core followed by mineral infiltration.
Uses & applications
Primarily of interest to collectors; historically used for stone tools due to sharp fracture edges; occasionally used in decorative landscaping.
Geological facts
Chert is so durable that it often survives as stream pebbles long after the surrounding limestone or shale has eroded away. The distinctive cracking pattern in this specimen resembles a miniature septarian concretion.
Field identification & locations
Identify by its hardness (surfaces won't be scratched by a steel knife) and fine-grained, smooth texture. Commonly found in riverbeds, gravel pits, and sedimentary outcrops.
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