sedimentary
Chert (specifically Gray Chert)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness: 7 on Mohs scale; Color: Dark gray to light gray with white weathering; Luster: Dull to waxy; Structure: Microcrystalline with conchoidal (shell-like) fracture; Cleavage: None.
- Hardness
- 7 on Mohs scale
- Color
- Dark gray to light gray with white weathering
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 on Mohs scale; Color: Dark gray to light gray with white weathering; Luster: Dull to waxy; Structure: Microcrystalline with conchoidal (shell-like) fracture; Cleavage: None.
Formation & geological history
Formed through the precipitation of silica from aqueous fluids or the accumulation of siliceous remains of organisms like diatoms and radiolarians; common in marine limestone and dolomite formations of various geological ages.
Uses & applications
Historically used for stone tools (arrowheads, scrapers) due to its sharp edges; used in modern times as road ballast, in glass manufacturing, and as a decorative stone in landscaping.
Geological facts
Chert is so hard it can scratch steel and glass. Because of its predictable, glass-like fracture pattern, it was the primary material for tool-making for prehistoric humans globally.
Field identification & locations
Identify in the field by its very smooth, scalloped fracture surfaces and superior hardness compared to limestone. It is commonly found as nodules or layers within sedimentary rock outcrops.
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