sedimentary

Chert (Flint) Nodule

Cryptocrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Tan, beige, or grey with a white chalky cortex; Luster: Waxy to dull; Crystal structure: Microcrystalline/Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Tan, beige, or grey with a white chalky cortex
Luster: Waxy to dull

Identified July 2, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed as nodules within sedimentary rocks like limestone or chalk through a chemical process where silica-rich fluids replace carbonate minerals. They often contain micro-fossil remains and date from various geological periods, commonly the Cretaceous.

Uses & applications

Historically used for stone tools and weapons due to its conchoidal fracture (producing sharp edges) and for generating sparks (flint and steel). Modern uses include road surfacing and some jewelry or lapidary work.

Geological facts

Chert nodes often have a white outer layer called a cortex, which is a weathered or porous layer of silica. When broken, it reveals a smooth, glass-like interior. It is one of the most durable rocks on the Earth's surface.

Field identification & locations

Identify in the field by looking for rounded, lumpy nodules in limestone outcrops. Use a scratch test (it will scratch glass) and look for the curved, shell-like (conchoidal) fracture pattern when broken. Commonly found in chalk downs or limestone regions.

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Sandstone or Siltstone

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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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