sedimentary
Chert (varieties: Flint or Jasper)
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Dark brown to black with reddish-tan weathering crust, Luster: Waxy to dull, Crystal Structure: Microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
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Physical properties
Hardness: 7 (Mohs scale), Color: Dark brown to black with reddish-tan weathering crust, Luster: Waxy to dull, Crystal Structure: Microcrystalline, Cleavage: None (conchoidal fracture), Specific Gravity: 2.65
Formation & geological history
Formed through the precipitation of silica-rich fluids in sedimentary environments (like limestone) or via the accumulation of siliceous organisms like diatoms and radiolarians. Often dates from the Paleozoic to Mesozoic eras.
Uses & applications
Historically used for making stone tools (knapping) and as strike-a-lights for fire making. Currently used in road surfacing and as a lapidary material for decorative stones/jewelry.
Geological facts
Because it breaks with a conchoidal fracture, it creates edges sharper than a surgeon's steel scalpel. Native Americans and early humans used these materials extensively for arrowheads and scrapers.
Field identification & locations
Identify in the field by its very hard surface (scratches glass), waxy luster, and shell-like (conchoidal) breaking pattern. Common in river beds and limestone outcrops.
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