sedimentary
Chert (Field-collected specimen)
Chert (Crystalline Silica, SiO2)
Hardness: 7 on Mohs scale; Color: Grayish-tan to brown; Luster: Dull to waxy; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Grayish-tan to brown
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Grayish-tan to brown; Luster: Dull to waxy; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of siliceous remains of marine organisms or the chemical precipitation of silica in sedimentary environments. Common in limestone and dolostone formations spanning from Precambrian to Cenozoic eras.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; used today as a decorative stone in landscaping, road aggregate, and by hobbyist flintknappers.
Geological facts
Chert is so hard it can easily scratch glass and will spark when struck against steel. It is one of the most durable rocks and often survives weathering long after the surrounding limestone has eroded.
Field identification & locations
Identify in the field by its extreme hardness, smooth conchoidal fracture (shell-like curves when broken), and lack of reactivity to acid. Common in riverbeds and as nodules in limestone outcrops.
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