sedimentary
Chert (Flint nodule)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale), Color: Dull gray, brown, and tan with a matte to waxy luster. Crystal structure: Trigonal (microscopic). Cleavage: None, exhibits characteristic conchoidal fracture. Specific gravity: 2.65.
- Color
- Dull gray, brown, and tan with a matte to waxy luster
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Physical properties
Hardness: 7 (Mohs scale), Color: Dull gray, brown, and tan with a matte to waxy luster. Crystal structure: Trigonal (microscopic). Cleavage: None, exhibits characteristic conchoidal fracture. Specific gravity: 2.65.
Formation & geological history
Formed through the replacement of limestone or chalk by silica-rich fluids. Often occurs as nodules or beds in sedimentary strata ranging from the Precambrian to the Cenozoic era. These specific nodules often form in deep-sea or carbonate environments.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; used in modern industry as a constituent in some concrete aggregates, abrasives, and historically for striking sparks in flintlock firearms.
Geological facts
Chert is so hard it can scratch steel. It was one of the first materials utilized by early humans for tools because it breaks with incredibly sharp, glass-like edges. Certain varieties with high iron content are referred to as Jasper.
Field identification & locations
Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass or pocketknife steel. Commonly found in limestone outcrops or as weathered-out nodules in gravel pits. Common in the Midwestern USA, UK (chalk cliffs), and various global sedimentary basins.
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