sedimentary

Chert (or Chalcedony)

Microcrystalline Quartz (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Cream, tan, or white with translucent patches; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.6-2.65.

Hardness: 6
Color: Cream, tan, or white with translucent patches
Luster: Waxy to vitreous

Identified June 18, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed in marine environments through the precipitation of silica from seawater or the accumulation of siliceous skeletal remains (like diatoms or radiolarians). It converts from opaline silica into microcrystalline quartz over millions of years.

Uses & applications

Historically used for stone tools (arrowheads/scrapers) due to its sharp conchoidal fracture. Modernly used as decorative stones, in lapidary work, and as a component in construction aggregate.

Geological facts

Because it is so hard and chemically stable, chert often remains as pebbles in stream beds long after the surrounding limestone or chalk has weathered away. It is virtually identical to flint, though usually lighter in color.

Field identification & locations

Identify in the field by its waxy appearance and the way it breaks into smooth, curved surfaces with sharp edges (conchoidal fracture). It will easily scratch glass but cannot be scratched by a steel knife. Common in limestone regions.

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Sandstone or Siltstone

Arenite (if sandstone)

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Chert Breccia

Brecciated Chert (Microcrystalline Silica)

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Sandstone

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Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

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Shale or Slaty Mudstone

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Sandstone

Arenite

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