sedimentary

Chert (Iron-Stained)

Microcrystalline or cryptocrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale), Color: Brownish-red to tan (due to iron oxide staining), Luster: Waxy to dull, Crystal structure: Hexagonal (microscopic), Cleavage: None (conchoidal fracture), Specific Gravity: 2.65

Identified March 1, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through a chemical process where silica-rich fluids replace organic matter or carbonate rocks, or through the accumulation of siliceous microorganisms (like radiolaria or diatoms) in marine environments. It can be found in sedimentary strata ranging from Precambrian to Cenozoic ages.

Uses & applications

Historically used for stone tools (lithics) due to its conchoidal fracture which creates sharp edges. Today used as road gravel, in construction as aggregate, and occasionally for lapidary work or aquarium decor.

Geological facts

Chert is so durable that it often remains on the Earth's surface long after the surrounding rock has eroded away. Iron-stained varieties can often be mistaken for jasper, which is a more opaque, pigmented variety of chert.

Field identification & locations

Identify in the field by its hardness (it will scratch glass), its waxy feel, and its shell-like (conchoidal) breaking pattern. It is commonly found in limestone or chalk beds. Collectors look for pieces with interesting color patterns or evidence of ancient tool-making (flakes).

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Sandstone or Siltstone

Arenite (if sandstone)

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Argillaceous sedimentary rock

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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Sandstone (Quartz-rich)

Arenite (SiO2)

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Sandstone (with man-made markings)

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