sedimentary
Chert (Yellow Jasper)
Silicon dioxide (SiO2) with iron oxide inclusions
Hardness: 6.5-7 (Mohs scale), Color: Mustard yellow with dark manganese/iron dendritic or mossy patterns, Luster: Waxy to dull, Structure: Cryptocrystalline, Fracture: Conchoidal
- Hardness
- 6
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Physical properties
Hardness: 6.5-7 (Mohs scale), Color: Mustard yellow with dark manganese/iron dendritic or mossy patterns, Luster: Waxy to dull, Structure: Cryptocrystalline, Fracture: Conchoidal
Formation & geological history
Formed from the accumulation of silica-rich microfossils (like radiolarians) or inorganic precipitation within sedimentary basins. The yellow color is due to goethite or limonite staining of the silica. The location (near Toulouse/Muret, France) is consistent with sedimentary deposits of the Aquitaine Basin.
Uses & applications
Used historically for stone tools due to its sharp conchoidal fracture. In modern times, it is used for lapidary work (cabochons), tumbled stones, and as a decorative landscaping rock.
Geological facts
Yellow chert is often termed 'Jasper' when opaque. It is extremely durable and chemical resistant, often surviving as river cobbles long after the softer surrounding rock has eroded away.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its smooth, waxy feel when broken. It is commonly found in riverbeds and gravel pits in the Garonne river valley region.
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