sedimentary
Chert (Chalcedony/Agate Pebble)
Microcrystalline Quartz (SiO2)
Hardness: 7 Mohs, Color: Tan, brown, and translucent white, Luster: Waxy to vitreous, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture)
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Physical properties
Hardness: 7 Mohs, Color: Tan, brown, and translucent white, Luster: Waxy to vitreous, Crystal structure: Cryptocrystalline, Cleavage: None (conchoidal fracture)
Formation & geological history
Formed from the accumulation of siliceous oozes or through the replacement of limestone by silica-rich fluids in sedimentary basins. Most are millions of years old, often rounded by water erosion.
Uses & applications
Historically used for stone tools and fire-starting. Currently used for gravel, aquarium decor, and small-scale lapidary work.
Geological facts
Chert was one of the most important minerals for early humans because its conchoidal fracture creates razor-sharp edges used for arrows and knives.
Field identification & locations
Can be identified by its extreme hardness (scratches glass), waxy feel, and translucent edges. Commonly found in river beds, beaches, and limestone outcrops.
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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