sedimentary
Chert (Field Stone)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, brown, gray, with some iron staining; Luster: Waxy to dull/earthy; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Tan, brown, gray, with some iron staining
- Luster
- Waxy to dull/earthy
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, brown, gray, with some iron staining; Luster: Waxy to dull/earthy; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by the chemical replacement of limestone/dolomite. It occurs in nodules or bedded layers within sedimentary rock sequences ranging from Precambrian to Quaternary in age.
Uses & applications
Historically used for stone tools and arrowheads because of its sharp conchoidal fracture. Modern uses include road aggregate, landscape stone, and occasionally used in jewelry when it displays unique patterns (Jasper).
Geological facts
Chert is so hard that it can scratch glass and steel. It was one of the first materials used by early humans to create fire-starting kits and hunting weapons. It is chemically identical to quartz but has a different internal grain structure.
Field identification & locations
Identify in the field by its extreme hardness, smooth waxy feel when broken, and lack of visible grains. It often has a weathered 'cortex' or chalky outer skin. Commonly found in riverbeds, limestone quarries, and gravel pits.
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