sedimentary

Chert (Black Flint)

Microcrystalline Quartz (SiO2)

AI-generated identification · may be incorrect

Mohs hardness of 7; color is opaque black to dark grey; waxy to dull luster; cryptocrystalline structure; prominent conchoidal fracture (breaks with shell-like curved surfaces); specific gravity 2.65.

Identified July 1, 2026More sedimentary →

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Physical properties

Formation & geological history

Formation occurs as nodules or layers in sedimentary rocks (like limestone) through the precipitation of silica from groundwater or the accumulation of siliceous biological remains (diatoms/radiolarians) on the ocean floor.

Uses & applications

Historically used for stone tools and fire-starting; currently used for decorative gravel, landscaping, and as a raw material for ceramics and glass when high purity.

Geological facts

Black flint gets its dark color from tiny inclusions of organic matter or carbon. It was the primary material for prehistoric survival tools because of its ability to hold a razor-sharp edge.

Field identification & locations

Identify by its smooth, waxy feel and the sharp edges formed by conchoidal fractures. It is commonly found in chalk or limestone outcrops and along coastal beaches where these rocks have eroded.