sedimentary
Chert (Iron-stained)
Silicon Dioxide (SiO2) with Ferruginous impurities
Hardness: 6.5-7 on Mohs scale; Color: Dark brown to yellow-brown; Luster: Dull to waxy; Structure: Microcrystalline/Cryptocrystalline; Fracture: Conchoidal; Specific Gravity: 2.6
- Hardness
- 6
- Color
- Dark brown to yellow-brown
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5-7 on Mohs scale; Color: Dark brown to yellow-brown; Luster: Dull to waxy; Structure: Microcrystalline/Cryptocrystalline; Fracture: Conchoidal; Specific Gravity: 2.6
Formation & geological history
Formed by the accumulation of siliceous remains of marine organisms or inorganic precipitation in limestone/chalk beds. The brown coloration comes from the oxidation of iron minerals (limonite/hematite) within the silicon matrix.
Uses & applications
Historically used for stone tools and starting fires (flint); currently used as road aggregate, in landscaping, or as a lapidary material for tumbling.
Geological facts
Chert is so durable that it often remains as gravel in riverbeds long after the surrounding limestone host rock has weathered away. It was one of the most important raw materials for prehistoric humans.
Field identification & locations
Identify by its extreme hardness (will scratch glass) and smooth, shell-like (conchoidal) fracture surfaces. Commonly found in nodules within sedimentary layers or as rounded pebbles in river deposits.
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