sedimentary
Chert (Iron-stained)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on the Mohs scale; Color: Reddish-brown, tan, and cream due to iron oxide impurities; Luster: Dull to waxy; Crystal structure: Trigonal (microscopic crystals); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on the Mohs scale
- Color
- Reddish-brown, tan, and cream due to iron oxide impurities
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on the Mohs scale; Color: Reddish-brown, tan, and cream due to iron oxide impurities; Luster: Dull to waxy; Crystal structure: Trigonal (microscopic crystals); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed through the accumulation of silica-rich microfossils (like radiolarians or diatoms) on the ocean floor or through chemical precipitation in groundwater. The reddish staining occurs as iron minerals like hematite or goethite leach into the porous silica matrix over millions of years.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp conchoidal fracture. Today used as road aggregate, in landscaping, and as a semi-precious gemstone (Jasper variety) for lapidary work.
Geological facts
Chert is so hard it can scratch steel. It was one of the most important materials for prehistoric humans because it breaks into razor-sharp edges. When struck against steel, it can produce sparks, leading to its frequent use in flintlock firearms.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its inability to be scratched by a knife blade. Commonly found in limestone or chalk beds as nodules or layers. Look for it in riverbeds or rocky outcrops globally.
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