sedimentary

Chert (and its variety Flint)

Silicon dioxide (SiO2) / Microcrystalline quartz

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

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

Identified March 15, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like diatoms or radiolarians) or by chemical precipitation from silica-rich fluids within existing sedimentary rock layers, often limestone or chalk.

Uses & applications

Historically used for stone tools (arrowheads/knives) due to sharp edges; commonly used today in road construction, as griding media, or as a gemstone if colorful (Jasper).

Geological facts

Because it breaks with a conchoidal fracture like glass, chert was the primary material for prehistoric 'flint-knapping.' It is also known for its ability to produce sparks when struck against steel.

Field identification & locations

Identify in the field by its smooth, waxy surface and its ability to scratch glass easily. Look for curved, shell-like (conchoidal) breaks on edges. Found globally in riverbeds, glacial tills, and embedded in limestone cliffs.

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Sandstone or Siltstone

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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)

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Sandstone (with man-made markings)

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