sedimentary

Chert (often called Flint when dark)

Microcrystalline or Cryptocrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Tan, beige, or off-white with some green/brown staining; Luster: Dull to waxy; Crystal Structure: Microcrystalline; Cleavage: None (shows conchoidal fracture); Specific Gravity: 2.6-2.7

Hardness: 7 on Mohs scale
Color: Tan, beige, or off-white with some green/brown staining
Luster: Dull to waxy

Identified May 8, 2026More sedimentary →

Explore Chert (often called Flint when dark) in the encyclopedia →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by the replacement of carbonate material in sedimentary beds. Most common in limestone or chalk deposits ranging from Precambrian to Cenozoic eras.

Uses & applications

Historically used for stone tools and weapons (arrowheads). Modernly used as an abrasive, in construction aggregate, and as a starting material for roadbeds.

Geological facts

Because it breaks with a conchoidal fracture, it creates edges sharper than a surgeon's scalpel. It was the primary material for the Stone Age technology revolution.

Field identification & locations

Identify by its hardness (it will scratch glass) and its smooth, curved 'shell-like' fracture patterns. Often found as nodules or layers within limestone. Collectors look for unique colors or fossil inclusions.

Other sedimentary specimens

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Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary