sedimentary
Chert (with Patina)
Microcrystalline Quartz (SiO2)
Hardness (Mohs scale 7), colors of gray, tan, and white; waxy to dull luster; cryptocrystalline structure; conchoidal fracture with sharp edges; high specific gravity (approx. 2.65).
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Physical properties
Hardness (Mohs scale 7), colors of gray, tan, and white; waxy to dull luster; cryptocrystalline structure; conchoidal fracture with sharp edges; high specific gravity (approx. 2.65).
Formation & geological history
Formed as chemical precipitates in deep marine environments or by the replacement of calcium carbonate in limestone by silica-rich fluids. Can be billions of years old up to relatively recent formations.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; used today as road ballast, in the manufacture of glass, and as decorative gravel.
Geological facts
Chert is so durable that it often survives as weathered pebbles long after the surrounding limestone has eroded away. The dark 'rind' or skin on the specimen is a weathering patina formed by mineral oxidation.
Field identification & locations
Identify in the field by its hardness (will scratch steel) and its shell-like (conchoidal) fracture pattern. Found globally in sedimentary basins and riverbeds.
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