sedimentary
Chert
Microcrystalline Quartz (SiO2)
AI-generated identification · may be incorrectMohs 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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Fossil-bearing concretion, possibly containing a fossil shell or other organic inclusion
Calcareous concretion; exact fossil and host-rock composition are not identifiable from the image
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Fossiliferous Sedimentary Rock
Sedimentary limestone with invertebrate fossil imprint
fossil
Sandstone or Siltstone
Arenite (if sandstone)
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Sandstone
Arenite
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Shale
Shale (clastic sedimentary rock)
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Shale (or Mudstone/Siltstone)
Sedimentary Mudrock (SiO2, Al2O3 base with clay minerals)
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