sedimentary

Chert (and Flint)

Cryptocrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: typically tan, grey, or brown; Luster: waxy to dull; Structure: microcrystalline (no visible grains); Cleavage: none, produces conchoidal (shell-like) fractures; Specific Gravity: 2.65.

Hardness: 7 on Mohs scale
Color: typically tan, grey, or brown
Luster: waxy to dull

Identified March 19, 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 the replacement of limestone/dolomite by silica-rich fluids. It is found in rock layers ranging from the Precambrian to the present.

Uses & applications

Historically used for stone tools (arrowheads, knives) due to its sharp edges; used in modern times as a road surface material, in grit for poultry, and occasionally in jewelry as semi-precious 'Jasper' or chalcedony.

Geological facts

Chert was a vital technological resource for prehistoric humans; striking it against steel or iron pyrites produces sparks, making it essential for early fire-starting and flintlock firearms.

Field identification & locations

Identify in the field by its hardness (will scratch glass), waxy feel, and sharp, curved fracture patterns. Commonly found as nodules or layers within limestone beds or as rounded pebbles in riverbeds.

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Sandstone or Siltstone

Arenite (if sandstone)

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Sandstone

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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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