sedimentary
Chert
Microcrystalline Quartz (SiO2)
Mohs hardness of 7, dark grey to olive-green color, dull to waxy luster, microcrystalline structure, and conchoidal fracture (curved, glass-like breaks).
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Physical properties
Mohs hardness of 7, dark grey to olive-green color, dull to waxy luster, microcrystalline structure, and conchoidal fracture (curved, glass-like breaks).
Formation & geological history
Formation occurs through the accumulation of silica-rich organic remains (like diatoms or radiolaria) on the ocean floor or through chemical precipitation. These beds lithify over millions of years.
Uses & applications
Historically used for making stone tools (arrowheads/knives) due to its sharp edges. Modernly used in landscaping and as a component in aggregate.
Geological facts
Because it is harder than steel, striking chert against steel will produce sparks, making it a historical 'strike-a-light' stone.
Field identification & locations
Identify in the field by its hardness (it will scratch glass) and its smooth, shell-like fracture surfaces. Frequently found as stream or beach pebbles.
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Cone-in-cone structure (Limestone)
Cone-in-cone structure (Secondary Sedimentary Structure)
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Sandstone Grain
Clastic Sedimentary Rock Grain (SiO2 dominated)
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Chert Breccia
Brecciated Chert (Microcrystalline Silica)
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Shale or Slaty Mudstone
Argillaceous sedimentary rock
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Sandstone
Arenite (primarily SiO2)
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