sedimentary
Chert (Iron-stained)
Silicon Dioxide (SiO2) with Iron Oxides
Hardness: 7 Mohs; Color: Brown, tan, rusty orange; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Fracture: Conchoidal; Specific Gravity: 2.65
- Hardness
- 7 Mohs
- Color
- Brown, tan, rusty orange
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 Mohs; Color: Brown, tan, rusty orange; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Fracture: Conchoidal; Specific Gravity: 2.65
Formation & geological history
Formed as nodules or layered deposits in limestone or dolostone through the accumulation of silica-rich organic remains (like radiolarians or sponge spicules) or chemical precipitation. The orange-brown color indicates secondary iron staining from groundwater.
Uses & applications
Historically used for making stone tools (arrowheads/scrapers) due to conchoidal fracture; modernly used as road base, decorative landscaping, or as a source of silica.
Geological facts
Chert is extremely durable and chemically resistant. It was one of the most important minerals for early humans because its edges can be sharper than a surgical scalpel when broken.
Field identification & locations
Identify by its 'flint-like' waxy texture, hardness (it will scratch glass), and the way it breaks into sharp, curved flakes. Found globally in sedimentary basins.
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