sedimentary
Brecciated Chert
Sedimentary Chert (SiO2) with Calc-Silicate mineralization
Hardness: 7 Mohs scale; Color: Mottled tan, brown, and dark grey; Luster: Dull to waxy; Structure: Microcrystalline with fractured angular clasts; Cleavage: None (conchoidal fracture).
- Hardness
- 7 Mohs scale
- Color
- Mottled tan, brown, and dark grey
- Luster
- Dull to waxy
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Physical properties
Hardness: 7 Mohs scale; Color: Mottled tan, brown, and dark grey; Luster: Dull to waxy; Structure: Microcrystalline with fractured angular clasts; Cleavage: None (conchoidal fracture).
Formation & geological history
Formed when silica-rich fluids permeate sedimentary rocks or replace limestone. The brecciated (broken) appearance occurs due to tectonic stress or shrinking during dehydration, followed by secondary mineral infilling.
Uses & applications
Used historically for stone tools due to sharp edges; currently used for road base, landscaping, and occasionally as lapidary material for jewelry cabochons.
Geological facts
Chert has been used by humans for hundreds of thousands of years to create arrowheads and scrapers. The dark spots and patterns often reveal the chemical history of the water that once flowed through the rock.
Field identification & locations
Identify in the field by its superior hardness (cannot be scratched by a steel knife) and its blocky, sharp-edged fractures. Commonly found in limestone outcrops or as river-worn cobbles.
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