sedimentary

Chert (Iron-stained)

Microcrystalline or cryptocrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale); Color: Varying shades of gray, brown, and reddish-orange due to iron oxidation; Luster: Waxy to dull; Crystal structure: Trigonal (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 (Mohs scale)
Luster: Waxy to dull

Identified March 29, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolaria or diatoms) or chemical precipitation in marine environments. This specimen likely dates from the Phanerozoic eon and has been tumbled in a river or coastal environment.

Uses & applications

Historically used for stone tools and weapons (flintknapping); currently used as a decorative stone, in construction aggregate, and occasionally as an abrasive.

Geological facts

Chert is so hard it can strike sparks against steel, a property that made it essential for early firearms (flintlocks). It is chemically stable and resistant to weathering, which is why it remains as cobbles in riverbeds long after other rocks have eroded.

Field identification & locations

Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and its ability to scratch glass. It is commonly found in gravel bars, river beds, and limestone deposits worldwide. Collectors look for unique colors or fossil inclusions.

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