sedimentary

Chert (specifically a variety like Flint or River Jasper with Iron Oxidation)

Microcrystalline Quartz (SiO2)

Hardness: 6.5–7 (Mohs scale); Color: Grayish-tan interior with red/orange iron-oxide weathering crust; Luster: Waxy to vitreous; Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 6
Luster: Waxy to vitreous

Identified February 16, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of silica from groundwater or the accumulation of siliceous skeletal remains of marine organisms. The orange/red exterior indicates secondary iron oxidation (rusting) from exposure to water and oxygen over thousands of years.

Uses & applications

Historically used for stone tools and weapons due to its sharp edges; currently used for road aggregate, landscaping, flint-and-steel fire making, and lapidary work (polishing/tumbling).

Geological facts

Chert is so hard it can scratch steel. When struck against iron or steel, it produces sparks, a property that led to its use in flintlock firearms and early fire-starting tools.

Field identification & locations

Identify in the field by its smooth, waxy feel and distinct 'conchoidal' (shell-like) fractures. It is frequently found in riverbeds, limestone outcrops, and coastal areas throughout North America and Europe.

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Sandstone or Siltstone

Arenite (if sandstone)

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Sandstone

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Shale or Slaty Mudstone

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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