sedimentary
Chert
Chert (SiO2)
Hardness: 7 (Mohs scale), Color: Brown, tan, or grey, Luster: Dull to waxy, Crystal Structure: Microcrystalline quartz, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
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Physical properties
Hardness: 7 (Mohs scale), Color: Brown, tan, or grey, Luster: Dull to waxy, Crystal Structure: Microcrystalline quartz, Cleavage: None (conchoidal fracture), Specific Gravity: 2.6-2.7
Formation & geological history
Formed primarily as chemical precipitates or biochemical deposits in deep marine environments or by the replacement of minerals in limestone and dolostone. Geological ages range from Precambrian to Recent.
Uses & applications
Historically used for stone tools and starting fires (flint). Used today as a hard-wearing construction aggregate and in landscaping.
Geological facts
Chert is so durable that it often remains in stream beds long after softer rocks have eroded away. Varieties include jasper, flint, and agate.
Field identification & locations
Identify by its smooth, waxy surface and conchoidal (shell-like) fractures. It will easily scratch glass and is commonly found in riverbeds and limestone outcrops.
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