sedimentary

Chert (specifically Jasper or Flint variety)

Cryptocrystalline Quartz (SiO2)

Hardness: 6.5-7 on the Mohs scale. Color: Waxy grey, tan, and reddish-brown due to iron oxide staining. Luster: Dull to waxy. Structure: Microcrystalline with distinct conchoidal (shell-like) fracture. Cleavage: None.

Hardness: 6
Color: Waxy grey, tan, and reddish-brown due to iron oxide staining
Luster: Dull to waxy

Identified March 25, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich microfossils or via chemical precipitation from silica-rich fluids within limestone or chalk beds. Often sedimentary in origin but can be associated with volcanic activity.

Uses & applications

Historically used for stone tools and arrowheads due to its sharp edges; modernly used as road aggregate, architectural stone, and semi-precious gemstones for cabochons and jewelry.

Geological facts

Chert was one of the most vital materials for early humans; its ability to hold a razor-sharp edge made it the primary choice for Neolithic survival tools. It can also produce sparks when struck against steel.

Field identification & locations

Identify in the field by its hardness (it will scratch glass) and its smooth, curved conchoidal fractures. Commonly found in riverbeds or within limestone outcrops. Found globally, extensively in the Midwestern and Western USA.

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Sandstone or Siltstone

Arenite (if sandstone)

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Sandstone

Arenite (primarily SiO2)

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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)

Arenite (composed primarily of Quartz, SiO2)

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