sedimentary
Chert (specifically Gray/Tan Bedded Chert)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale (scratches glass). Color: Gray, tan, and creamy white with earthy patches. Luster: Waxy to dull. Crystal structure: Microcrystalline (individual crystals not visible to eye). Cleavage: None, exhibits conchoidal (shell-like) fracture. Specific gravity: 2.65.
- Hardness
- 7 on Mohs scale (scratches glass)
- Color
- Gray, tan, and creamy white with earthy patches
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 on Mohs scale (scratches glass). Color: Gray, tan, and creamy white with earthy patches. Luster: Waxy to dull. Crystal structure: Microcrystalline (individual crystals not visible to eye). Cleavage: None, exhibits conchoidal (shell-like) fracture. Specific gravity: 2.65.
Formation & geological history
Formed from the precipitation of silica-rich fluids in sedimentary basins. Often originates from the accumulation of silica-based skeletal remains of microscopic marine organisms (like radiolarians or diatoms) that recrystallize over millions of years into a dense rock.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges when broken. In modern times, used as an abrasive, in road construction as aggregate, and occasionally for lapidary art.
Geological facts
Chert is so durable that it often remains as gravel long after the surrounding limestone or parent rock has weathered away. It was one of the first materials used by early humans to create fire and weaponry.
Field identification & locations
Identify in the field by its hardness (it cannot be scratched by a steel knife) and its curved, glass-like fracture surfaces. It often occurs as nodules or distinct beds within limestone or chalk formations. Common globally.
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