sedimentary
Chert (Jasper/Flint variety)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 6.5–7 on Mohs scale. Color: Highly variable, featuring dark grey, brown, tan, and reddish-orange hues (likely due to iron oxide impurities). Luster: Dull to waxy. Crystal structure: Cryptocrystalline. Cleavage: None, exhibits sharp conchoidal (shell-like) fracture patterns.
- Hardness
- 6
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5–7 on Mohs scale. Color: Highly variable, featuring dark grey, brown, tan, and reddish-orange hues (likely due to iron oxide impurities). Luster: Dull to waxy. Crystal structure: Cryptocrystalline. Cleavage: None, exhibits sharp conchoidal (shell-like) fracture patterns.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or via chemical precipitation from silica-rich groundwater replacing other minerals/sediments. Many deposits range from the Cambrian to the Cretaceous periods.
Uses & applications
Historically used for stone tools and weapons due to its ability to hold a sharp edge. Modernly used as high-end road gravel, in landscaping for decorative garden rocks, as a silica source in glassmaking, and occasionally for lapidary work (cabochons).
Geological facts
Chert is so durable that it can survive long-distance transport in rivers and survive being tumbled on beaches. It was the primary 'high-tech' material of the Stone Age, used globally for spearheads, arrowheads, and scrapers.
Field identification & locations
Identify in the field by its hardness (it will easily scratch glass and cannot be scratched by a steel knife) and its curved, sharp-edged conchoidal fractures. It is commonly found in limestone and chalk formations as nodules or layered beds. Look for it in riverbeds and limestone outcrops.
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