sedimentary
Chert (specifically Banded Chert)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 6.5–7 on Mohs scale. Color: Gray, tan, or white with reddish-brown iron oxide bands. Luster: Waxy to dull. Structure: Microcrystalline with conchoidal (shell-like) fracture. Specific gravity: 2.6–2.7.
- Hardness
- 6
- Color
- Gray, tan, or white with reddish-brown iron oxide bands
- Luster
- Waxy to dull
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Physical properties
Hardness: 6.5–7 on Mohs scale. Color: Gray, tan, or white with reddish-brown iron oxide bands. Luster: Waxy to dull. Structure: Microcrystalline with conchoidal (shell-like) fracture. Specific gravity: 2.6–2.7.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or chemical precipitation from silica-rich fluids within sedimentary basins. Can range from Precambrian to Holocene in age.
Uses & applications
Historically used for stone tools (arrowheads/scrapers) due to sharp edges; currently used for road base, decorative landscaping, and occasionally as an abrasive or in glass manufacturing.
Geological facts
Chert was one of the most important minerals for early human survival because its conchoidal fracture creates edges sharper than a steel scalpel. The red banding is often caused by trace amounts of hematite.
Field identification & locations
Identify in the field by its hardness (it will scratch glass), waxy feel, and sharp, curved edges when broken. Commonly found in limestone beds or as loose nodules in riverbeds across North America and Europe.
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