sedimentary
Chert (Limonite-stained)
Microcrystalline Quartz (SiO2) with iron oxides
Hardness: 7 (Mohs scale), Color: Yellow-brown to tan (due to limonite), Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture)
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Physical properties
Hardness: 7 (Mohs scale), Color: Yellow-brown to tan (due to limonite), Luster: Waxy to dull, Crystal Structure: Cryptocrystalline, Cleavage: None (conchoidal fracture)
Formation & geological history
Formed from the accumulation of siliceous remains of marine organisms or through chemical precipitation in sedimentary environments. Its yellow hue comes from iron oxidation.
Uses & applications
Historically used for stone tools (flintknapping); currently used in road gravel, decorative landscaping, and as a polishing material in rock tumblers.
Geological facts
Chert is one of the most durable sedimentary rocks. Highly prized 'Fairburn Agates' are a specific variety of banded chert found in the Great Plains.
Field identification & locations
Identify by its extreme hardness (scratches glass), waxy feel, and smooth curved (conchoidal) fractures where chips have come off. Common in riverbeds and glacial till.
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