sedimentary
Chert (and Jasper pebble)
Microcrystalline Silica (SiO2)
Hardness: 7 (Mohs scale); Color: Gray-blue to olive green; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 – 2.7.
- Hardness
- 7 (Mohs scale)
- Color
- Gray-blue to olive green
- Luster
- Waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: Gray-blue to olive green; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 – 2.7.
Formation & geological history
Formed through the accumulation of silica-rich organic remains (like diatoms or radiolaria) or chemical precipitation in marine environments. These specimens appear to be river-worn or beach-tumbled pebbles.
Uses & applications
Used historically for stone tools and weapons (flint knapping); currently used in road construction as gravel, for jewelry (when colorful/polished), and as abrasive material.
Geological facts
Chert is so hard it can create sparks when struck against steel. Prehistoric civilizations traded specific types of high-quality chert over hundreds of miles to make arrowheads and knives.
Field identification & locations
Identify by its 'shell-like' conchoidal fracture patterns and inability to be scratched by a steel pocket knife. Commonly found in stream beds, glacial till, and embedded in limestone layers.
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Cone-in-cone structure (Secondary Sedimentary Structure)
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Clastic Sedimentary Rock Grain (SiO2 dominated)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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Shale or Slaty Mudstone
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Sandstone
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