sedimentary
Chert
Microcrystalline quartz (SiO2)
Hardness of 7 on Mohs scale, waxy to dull luster, conchoidal fracture, colors range from white to gray, brown, and black, cryptocrystalline structure
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Physical properties
Hardness of 7 on Mohs scale, waxy to dull luster, conchoidal fracture, colors range from white to gray, brown, and black, cryptocrystalline structure
Formation & geological history
Formed through the accumulation of siliceous skeletal remains of marine organisms like radiolaria and diatoms, or through chemical replacement of limestone by silica-rich groundwater
Uses & applications
Historically used by early humans for making stone tools, arrowheads, and spark generation with steel; occasionally used for decorative gravel or ornamental items
Geological facts
Chert is extremely durable and chemically resistant, often weathering out of limestone formations as nodules and surviving long-distance transport in riverbeds
Field identification & locations
Identified easily by its extreme hardness, smooth conchoidal fracture, and lack of reaction to hydrochloric acid; found worldwide in sedimentary rock formations
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