sedimentary

Chert (Flint with Iron Staining)

Silicon Dioxide (SiO2)

Hardness: 7 (Mohs scale); Color: White, gray, and brown-red due to iron oxidation; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.7

Hardness: 7 (Mohs scale)
Color: White, gray, and brown-red due to iron oxidation
Luster: Waxy to dull

Identified July 1, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by chemical precipitation in sedimentary basins. Often found as nodules in limestone or chalk beds.

Uses & applications

Historically used for stone tools and weapons due to its sharp edges; currently used as construction aggregate, road surfacing, and as a raw material in some ceramics.

Geological facts

Chert was one of the most important minerals for early humans, used to create arrowheads and spears. The red coloring in this specific piece is likely caused by the oxidation of iron minerals (hematite or limonite) coating the silica.

Field identification & locations

Identifiable by its smooth, waxy fracture surfaces and extreme hardness (it will scratch glass). Commonly found in riverbeds, fields, and within limestone outcroppings. Look for the characteristic conchoidal fracture patterns.

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Sandstone or Siltstone

Arenite (if sandstone)

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Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

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Shale or Slaty Mudstone

Argillaceous sedimentary rock

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Sandstone

Arenite (primarily SiO2)

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