sedimentary

Chert (Iron-stained)

Chert (SiO2 - Silicon Dioxide)

Hardness: 7 on Mohs scale; Color: Variable (this specimen shows reddish-brown iron staining and gray interior); Luster: Dull to waxy; Structure: Microcrystalline/cryptocrystalline; Fracture: Conchoidal (shell-like curves); Specific Gravity: 2.65.

Hardness: 7 on Mohs scale
Luster: Dull to waxy

Identified March 22, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of silica in marine environments, often replacing calcium carbonate in limestone or through the accumulation of siliceous organisms like diatoms and radiolarians. Found in formations ranging from the Precambrian to the Cenozoic era.

Uses & applications

Historically used for stone tools and weapons (arrowheads) due to sharp edges; currently used as construction aggregate, road base, and occasionally in lapidary work as jasper if colorful.

Geological facts

Chert is so hard it can create a spark when struck against steel. It is nearly chemically inert and resists weathering, which is why it often remains as large, angular chunks in soil.

Field identification & locations

Identify in the field by its superior hardness (it will scratch glass and steel), conchoidal fracture patterns, and lack of visible grains. Commonly found in creek beds or as nodules protruding from limestone outcrops.

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