sedimentary
Chert (Yellow/Brown Jasper)
Silicon Dioxide (SiO2)
Hardness: 6.5–7 (Mohs scale); Color: Yellowish-brown, tan, with darker iron oxide staining; Luster: Dull to waxy; Structure: Microcrystalline (cryptocrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6–2.7
- Hardness
- 6
- Color
- Yellowish-brown, tan, with darker iron oxide staining
- Luster
- Dull to waxy
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Physical properties
Hardness: 6.5–7 (Mohs scale); Color: Yellowish-brown, tan, with darker iron oxide staining; Luster: Dull to waxy; Structure: Microcrystalline (cryptocrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6–2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or by chemical precipitation from silica-rich fluids in sedimentary rock voids. Often found in limestone or as water-worn pebbles in riverbeds.
Uses & applications
Historically used for stone tools and arrowheads due to its sharp edges when fractured. Modern uses include decorative gravel, lapidary work (polishing for jewelry like necklaces or cabochons), and as a gemstone component.
Geological facts
Chert is incredibly durable and resistant to chemical weathering. Early humans prized this specific rock type because it fractures predictably into razor-sharp edges, a property known as conchoidal fracturing.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its smooth, waxy surface when broken. It often feels 'heavier' than common limestone. Commonly found in riverbeds, glacial tills, and coastal areas throughout North America and Europe.
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