sedimentary
Chert
Microcrystalline Quartz (SiO2)
Hardness 7 on Mohs scale, earthy to dull or waxy luster, conchoidal fracture, variable brown, tan, and grey colors, non-porous and cryptocrystalline texture
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Physical properties
Hardness 7 on Mohs scale, earthy to dull or waxy luster, conchoidal fracture, variable brown, tan, and grey colors, non-porous and cryptocrystalline texture
Formation & geological history
Formed by the precipitation of silica from groundwater or biological accumulation of siliceous organism remains (such as radiolarians or diatoms) in marine sediments.
Uses & applications
Historically used by ancient cultures for knapping tools, arrowheads, and fire-starting flints. Currently used as construction aggregate and landscaping gravel.
Geological facts
Chert nodules often form inside limestone or chalk beds. Dark grey to black varieties of chert are commonly referred to as flint.
Field identification & locations
Identified by its high hardness (scratches glass), smooth conchoidal fracturing, and non-reactivity with dilute acid (unlike surrounding limestone). Found worldwide in sedimentary rock formations.
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