sedimentary
Chert (Nodule)
Silicon dioxide (SiO2) - Cryptocrystalline Quartz
Hardness: 7 (Mohs scale); Color: Tan, brown, or light grey; Luster: Waxy to dull; Crystal structure: Microcrystalline (trigonal); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Tan, brown, or light grey
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan, brown, or light grey; Luster: Waxy to dull; Crystal structure: Microcrystalline (trigonal); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich fluids within limestone or chalk beds. Often resulting from the dissolution of siliceous skeletal remains (like sponges or diatoms) that re-precipitate as nodules. Typically found in Paleozoic to Mesozoic sedimentary strata.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to sharp edges; currently used in industrial abrasives, road construction aggregate, and as a decorative landscaping rock.
Geological facts
Chert is a chemical sedimentary rock. It is so hard that it can scratch steel and produce sparks when struck against iron, a property that led to its use in flintlock firearms.
Field identification & locations
Identify in the field by its very smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. It is commonly found in riverbeds or eroding out of limestone cliffs across the Midwestern US and Europe.
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