sedimentary
Chert
Microcrystalline Quartz (SiO2)
Hardness: 7 (Mohs scale); Color: gray, tan, or brown; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.5-2.8
- Hardness
- 7 (Mohs scale)
- Color
- gray, tan, or brown
- Luster
- waxy to dull
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Physical properties
Hardness: 7 (Mohs scale); Color: gray, tan, or brown; Luster: waxy to dull; Crystal structure: cryptocrystalline; Cleavage: none (conchoidal fracture); Specific gravity: 2.5-2.8
Formation & geological history
Formed by the accumulation of siliceous oozes on the seafloor or by the replacement of carbonate minerals in limestone or chalk beds. Often Paleozoic to Mesozoic in age.
Uses & applications
Historically used for stone tools (arrowheads). Modern uses include road surfacing, construction aggregate, and as a polishing media in rock tumblers.
Geological facts
Flint is a variety of chert. Because of its hardness and conchoidal fracture, it was one of the most important materials for early humans to create sharp cutting tools and fire-starters.
Field identification & locations
Identify in the field by its hardness (it will scratch glass), waxy feel, and smooth, curved fracture surfaces. Commonly found in limestone outcrops or as rounded pebbles in riverbeds.
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Sandstone or Siltstone
Arenite (if sandstone)
Sedimentary
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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