sedimentary

Chert Nodule (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Tan/brown weathered cortex with dark grey/black interior; Luster: Waxy to vitreous; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Tan/brown weathered cortex with dark grey/black interior
Luster: Waxy to vitreous

Identified April 6, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed by the precipitation of silica within marine sedimentary rocks (like limestone or chalk). It often concentrates around organic nuclei. They can range from Precambrian to recent, but are very common in Cretaceous formations.

Uses & applications

Historically used for stone tools (arrowheads, knives) and strike-a-lights due to sparks; currently used in industrial abrasives, road construction, and for decorative landscaping or historical tool reconstruction.

Geological facts

Flint was one of the most important resources for prehistoric humans because of its predictable conchoidal fracture which creates razor-sharp edges. It is essentially a very dense form of microcrystalline quartz.

Field identification & locations

Identify in the field by its rounded or knobby 'potato-like' exterior (cortex), high hardness (will scratch glass), and the characteristic dark, glassy interior when broken. Look for it in chalk cliffs or limestone basins.

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