sedimentary
Chert (specifically Flint or Common Chert)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Gray, green, brown, or white; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale
- Color
- Gray, green, brown, or white
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Gray, green, brown, or white; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.
Formation & geological history
Formed from the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation in sedimentary environments such as limestone or chalk beds. Most specimens found today date from the Paleozoic to Mesozoic eras.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to its sharp edges; used in construction aggregate and as a spark-generator in flintlock firearms.
Geological facts
Chert is so hard it can scratch steel. When struck against steel or iron pyrites, it produces sparks, which led to its vital historical role in starting fires and firing primitive guns.
Field identification & locations
Identify in the field by its smooth, waxy surface and characteristic shell-like (conchoidal) curved fractures. It is commonly found as nodules in limestone outcrops or as river-worn pebbles.
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