sedimentary
Chert (Nodular Chert)
Microcrystalline Silica (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, brown, or tan with white inclusions; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (it has conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, brown, or tan with white inclusions
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, brown, or tan with white inclusions; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (it has conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed by the precipitation of silica from groundwater within sedimentary rock layers, often limestone or chalk. Many nodules originate from the accumulation of silica-rich skeletal remains of marine organisms like sponges or diatoms during the Phanerozoic eon.
Uses & applications
Historically used for stone tools and weapons due to its sharp edges when fractured. Today, it is used as road metal, in construction aggregate, and as a component in certain ceramics. Higher quality specimens may be used as gemstones in lapidary work.
Geological facts
Chert is so hard it can scratch steel and glass. Because it fractures in a predictable conchoidal way, it was one of the most important minerals for ancient humans to create arrowheads and scrapers. It is also often associated with flint, which is a dark variety of chert.
Field identification & locations
Identify in the field by looking for a smooth, waxy surface and checking for conchoidal (shell-like) fracture patterns. It will typically scratch a steel knife blade. It is commonly found in riverbeds or eroding out of limestone cliffs worldwide.
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