sedimentary

Chert / Flint Nodules

Cryptocrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale); Color: Typically tan, brown, or gray with possible banding; Luster: Waxy to dull; Crystal structure: Microcrystalline (no visible grains); Cleavage: None (conchoidal fracture); Specific Gravity: 2.65.

Hardness: 7 (Mohs scale)
Color: Typically tan, brown, or gray with possible banding
Luster: Waxy to dull

Identified March 25, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or sponges) or chemical precipitation in marine environments. Often found as nodules within limestone or chalk beds, typically dating from the Paleozoic to Mesozoic eras.

Uses & applications

Historically used for stone tools and starting fires; modern uses include construction aggregate, road surfacing, and as a raw material for ceramics and glass.

Geological facts

Chert was the primary material for prehistoric humanity's tools because it flakes into extremely sharp edges. It is often referred to as 'earth's natural glass' and can generate sparks when struck against steel.

Field identification & locations

Identify in the field by its smooth, waxy feel and circular 'conchoidal' fractures that look like shell-shaped chips. Commonly found in riverbeds or eroded limestone outcrops. It will easily scratch glass and steel.

Other sedimentary specimens

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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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