sedimentary

Chert

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Brownish-gray, tan; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 on Mohs scale
Color: Brownish-gray, tan
Luster: Waxy to dull

Identified June 26, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Brownish-gray, tan; Luster: Waxy to dull; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed by the precipitation of silica from groundwater in sedimentary environments, specifically within carbonate rocks like limestone and dolomite. Typical geological age in the Iowa/Illinois region corresponds to the Paleozoic era (Silurian or Devonian).

Uses & applications

Commonly used in road construction as aggregate; historically used for stone tools (arrowheads) due to its sharp edges; used in aquariums and landscaping.

Geological facts

This specimen is highly likely part of the Galena or Maquoketa formations common in Eastern Iowa. When struck against steel, chert can produce sparks.

Field identification & locations

Identified by its waxy appearance, extreme hardness (scratches glass), and shell-like curved (conchoidal) fractures. Found abundantly in glacial drift and riverbeds in the Upper Mississippi River Valley.