sedimentary
Chert (Iron-stained)
Microcrystalline Quartz (SiO2)
Hardness: 7.0 (Mohs scale), Color: White, tan, and reddish-brown (from iron oxidation), Luster: Waxy to vitreous, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
- Hardness
- 7
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Physical properties
Hardness: 7.0 (Mohs scale), Color: White, tan, and reddish-brown (from iron oxidation), Luster: Waxy to vitreous, Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation from groundwater within limestone or chalk beds. This specimen displays secondary iron staining and weathering.
Uses & applications
Historically used for stone tools/arrowheads due to its sharp edges. Modern use includes landscaping, concrete aggregate, and occasional lapidary work for tumbled stones.
Geological facts
Chert is so durable that it often remains as 'river gravel' long after the surrounding limestone bedrock has eroded away. It was one of the most important minerals for prehistoric humans because it sparks when struck against steel or pyrite.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in stream beds, gravel pits, and as nodules in sedimentary outcrops.
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