sedimentary
Chert (Nodule)
Silicon dioxide (SiO2) - Cryptocrystalline Quartz
Hardness: 7 (Mohs scale); Color: Tan, beige, or brownish-grey; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 (Mohs scale)
- Color
- Tan, beige, or brownish-grey
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Tan, beige, or brownish-grey; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the precipitation of silica-rich fluids within sedimentary rock layers, primarily limestone or chalk. Many nodules originate from the accumulation of silica-based skeletal remains of sea organisms like diatoms or radiolarians during the Paleozoic to Cenozoic eras.
Uses & applications
Used historically for stone tools and arrowheads due to its sharp edges; and modernly in construction aggregate, road surfacing, and occasionally as a gemstone (when colorful, known as jasper).
Geological facts
Chert was one of the most important minerals for early humans because its conchoidal fracture allows it to be shaped into blades sharper than a surgical scalpel. It is often referred to as flint when found in chalk deposits.
Field identification & locations
Identify in the field by its smooth, waxy surface and its ability to scratch glass. It often feels denser than the surrounding limestone. Commonly found in riverbeds, limestone outcrops, and coastal areas throughout North America and Europe.
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