sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale), Color: tan, brown, or grayish with waxy luster, Crystal structure: trigonal (microcrystalline), Cleavage: none (conchoidal fracture), Specific gravity: 2.6-2.7.

Identified January 5, 2026More sedimentary →

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Physical properties

Hardness: 7 (Mohs scale), Color: tan, brown, or grayish with waxy luster, Crystal structure: trigonal (microcrystalline), Cleavage: none (conchoidal fracture), Specific gravity: 2.6-2.7.

Formation & geological history

Formed via the accumulation of silica-rich organic debris (like sponges or radiolarians) or chemical precipitation from groundwater within limestone or chalk beds, primarily during the Cretaceous or older periods.

Uses & applications

Historically used for stone tools (knapping) and fire-starting; currently used in road construction, as an abrasive, and sometimes as semi-precious gemstones or architectural veneer.

Geological facts

Flint is a variety of chert that occurs in chalk or marly limestone; it was the primary material for spearheads and arrowheads during the Stone Age because of its predictable fracture patterns.

Field identification & locations

Identify in the field by its conchoidal (shell-like) fracture edges, waxy feel, and its ability to scratch glass; often found as nodules in chalk cliffs or as rounded pebbles in riverbeds.