sedimentary
Banded Chert
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs), Color: Brown, tan, and cream banding, Luster: Waxy to dull, Crystal structure: Trigonal (microcrystalline/cryptocrystalline), Cleavage: None, splintery or conchoidal fracture
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Physical properties
Hardness: 7 (Mohs), Color: Brown, tan, and cream banding, Luster: Waxy to dull, Crystal structure: Trigonal (microcrystalline/cryptocrystalline), Cleavage: None, splintery or conchoidal fracture
Formation & geological history
Biogenic or chemical precipitation of silica in marine environments. Often formed from the skeletal remains of diatoms or radiolarians, or via replacement of limestone.
Uses & applications
Historically used for making stone tools (flint knapping); currently used as a decorative stone, in aquarium gravel, and occasionally as a minor gemstone when polished.
Geological facts
Chert is so hard it can create sparks when struck against steel. Its banding often records changes in the depositional environment over thousands of years.
Field identification & locations
Identify in the field by its superior hardness (scratches glass), waxy luster, and banding. Often found as nodules within limestone or as bedded layers in sedimentary outcrops.
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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