sedimentary

Chert (or Chalcedony)

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Tan, brown, white, and translucent gray; Luster: Waxy to vitreous; Crystal structure: Trigonal/Microcrystalline; Cleavage: None (conchoidal fracture); Specific Gravity: 2.65

Hardness: 7 on Mohs scale
Color: Tan, brown, white, and translucent gray
Luster: Waxy to vitreous

Identified July 1, 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 chemical precipitation of silica in groundwater within sedimentary host rocks. It can occur as nodules or bedded layers.

Uses & applications

Historically used for making stone tools due to its sharp edges; used today as a decorative landscaping stone, in construction aggregate, and occasionally for folk jewelry or as a collective specimen.

Geological facts

Chert has been used by humans for millions of years to create arrowheads and scrapers. When struck against steel, it can produce sparks, and it is a hard, durable material that resists weathering.

Field identification & locations

Identified in the field by its waxy texture, extreme hardness (it will scratch glass), and its characteristic shell-like (conchoidal) fracture pattern. Commonly found in limestone or chalk beds and along riverbed gravels.

Other sedimentary specimens

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Sandstone or Siltstone

Arenite (if sandstone)

Sedimentary

Cone-in-cone structure (Limestone)

Cone-in-cone structure (Secondary Sedimentary Structure)

sedimentary

Chert Breccia

Brecciated Chert (Microcrystalline Silica)

sedimentary

Sandstone Grain

Clastic Sedimentary Rock Grain (SiO2 dominated)

sedimentary

Shale or Slaty Mudstone

Argillaceous sedimentary rock

sedimentary

Sandstone

Arenite (primarily SiO2)

sedimentary