sedimentary
Chert (Nodule)
Silicon Dioxide (SiO2)
Hardness: 7 (Mohs scale); Color: Gray, tan, and brown with white inclusions; Luster: Dull to waxy; Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
- Hardness
- 7 (Mohs scale)
- Color
- Gray, tan, and brown with white inclusions
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray, tan, and brown with white inclusions; Luster: Dull to waxy; Structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or chemical precipitation from groundwater within limestone or chalk beds. Often Cretaceous to Paleozoic in age.
Uses & applications
Historically used for stone tools and weapons. Currently used in road surfacing, construction aggregate, and occasionally as lapidary material for tumbling or cabochons.
Geological facts
Chert is so hard it can scratch steel and produce sparks when struck against iron, a property that led to its widespread use in flintlock firearms. When it is high-quality and colorful, it is often called Jasper.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. Commonly found as nodules or layers within sedimentary rock outcrops like limestone.
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