sedimentary

Chert (Septarian or Nodular variety)

Microcrystalline Quartz (SiO2)

Hardness: 6.5 - 7 on Mohs scale; Color: Dark grey to charcoal with tan/brown veining; Luster: Waxy to dull (earthy); Crystal Structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.6 - 2.7

Hardness: 6
Color: Dark grey to charcoal with tan/brown veining
Luster: Waxy to dull (earthy)

Identified March 25, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the precipitation of silica in sedimentary rock layers, often replacing organic material or filling cavities in limestone or shale. The veined appearance suggests secondary mineral infilling of cracks caused by dehydration and shrinkage (septarian-like process).

Uses & applications

Primarily used as a lapidary material for cabochons or decorative stones; historically used for stone tools due to its sharp conchoidal fracture. Sometimes used as construction aggregate.

Geological facts

Chert is a chemical sedimentary rock. Dark varieties like this are often referred to as 'Flint' in certain geological contexts. It is extremely durable and can remain intact for millions of years while other surrounding rocks erode.

Field identification & locations

Identify in the field by its hardness (will scratch glass) and its smooth, shell-like (conchoidal) fracture surfaces. Commonly found in riverbeds, glacial tills, and limestone outcrops throughout North America and Europe.

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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)

sedimentary