sedimentary
Chert (variations include Flint/Jasper)
Microcrystalline or Cryptocrystalline Quartz (SiO2)
Hardness (7 on Mohs scale), Color: tan, grey, brown, white; Luster: waxy to dull; Crystal structure: hexagonal (microscopic); Cleavage: none (conchoidal fracture); Specific gravity: 2.6-2.7
- Color
- tan, grey, brown, white
- Luster
- waxy to dull
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Physical properties
Hardness (7 on Mohs scale), Color: tan, grey, brown, white; Luster: waxy to dull; Crystal structure: hexagonal (microscopic); Cleavage: none (conchoidal fracture); Specific gravity: 2.6-2.7
Formation & geological history
Formed by the accumulation of siliceous remains of microscopic organisms (like diatoms or radiolarians) or through chemical precipitation from silica-rich fluids in sedimentary basins. Often found as nodules or layers within limestone or chalk.
Uses & applications
Historically used for making stone tools (arrowheads, scrapers) due to conchoidal fracture. Today used as road aggregate, in landscaping, and some high-quality varieties are used in lapidary work.
Geological facts
Chert has been used by humans for over 2 million years. It is chemically identical to quartz but has an entirely different structural texture. Flint is a specific variety of chert typically found in chalk deposits.
Field identification & locations
Identify in the field by its hardness (scratches steel/glass), smooth conchoidal fracture edges (sharp like glass), and waxy appearance. Commonly found in stream beds and outcropping from limestone cliffs.
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Cone-in-cone structure (Limestone)
Cone-in-cone structure (Secondary Sedimentary Structure)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
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Shale or Slaty Mudstone
Argillaceous sedimentary rock
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Sandstone
Arenite (primarily SiO2)
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