sedimentary
Chert (Field Stone)
Microcrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Tan, brown, and grey with iron staining; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
- Hardness
- 7 on Mohs scale
- Color
- Tan, brown, and grey with iron staining
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, brown, and grey with iron staining; Luster: Waxy to dull; Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6-2.7
Formation & geological history
Formed through the accumulation of silica-rich organic remains or chemical precipitation in sedimentary environments. Large nodules often weather out of limestone or chalk beds.
Uses & applications
Historically used for stone tools and starting fires (flint); currently used as road aggregate, decorative landscaping stone, and occasionally for lapidary tumbling.
Geological facts
Chert is so durable that it often survives millions of years of erosion, eventually appearing as rounded 'river rocks' or field stones far from its original source rock.
Field identification & locations
Identified by its extreme hardness (cannot be scratched by a steel knife) and its smooth, shell-like fracture pattern. Commonly found in gravel pits, riverbeds, and fields.
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