sedimentary

Chert (specifically Black Chert / Flint)

Silicon Dioxide (SiO2)

Hardness: 7 on Mohs scale; Color: Dark grey to black; Luster: Waxy to dull; Crystal structure: Cryptocrystalline (microscopic quartz); Cleavage: None (exhibits conchoidal fracture); Specific gravity: 2.65.

Hardness: 7 on Mohs scale
Color: Dark grey to black
Luster: Waxy to dull

Identified March 14, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of siliceous remains of microscopic organisms (like diatoms or radiolarians) or via chemical precipitation of silica in marine environments. It occurs as nodules or layered beds in limestone and chalk deposits.

Uses & applications

Historically used for stone tools (arrowheads, knives) due to its sharp edges; used in flintlock firearms; currently used as road aggregate, in glass making, or as decorative landscaping stone.

Geological facts

Chert is so hard that it can scratch steel. When struck against iron or steel, it produces sparks, which led to its famous use as a fire-starter and in early weaponry. It is one of the most durable rocks on the planet.

Field identification & locations

Identify in the field by its smooth, shell-like (conchoidal) fracture surfaces and sharp edges when broken. It cannot be scratched by a steel knife. Common in sedimentary basins worldwide, particularly in areas with limestone outcrops.

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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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