sedimentary
Chert (Field Stone)
Chert (Silicon Dioxide, SiO2)
Hardness: 7 on Mohs scale; Color: Tan, beige, or light brown with potential iron staining; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
- Hardness
- 7 on Mohs scale
- Color
- Tan, beige, or light brown with potential iron staining
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Tan, beige, or light brown with potential iron staining; Luster: Dull to waxy; Structure: Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or chemical precipitation in sedimentary environments. Common in limestone and chalk beds.
Uses & applications
Historically used for making stone tools (arrowheads/scrapers) due to conchoidal fracturing. Modern uses include road aggregate and landscaping; high-quality varieties like flint can be used as fire starters.
Geological facts
Chert is a cryptocrystalline variety of quartz. It has been used by humans for hundreds of thousands of years to create sharp-edged tools because it breaks like glass but is much harder.
Field identification & locations
Look for a glassy/waxy texture when broken and a sharp edge. It will easily scratch glass. Commonly found in gravel pits, riverbeds, and as nodules within limestone outcrops.
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