sedimentary

Chert (Tan/Grey variety)

Silicon Dioxide (SiO2)

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

Hardness: 7 on Mohs scale
Color: Tan, grey, white, or cream
Luster: Waxy to dull

Identified March 8, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the accumulation of silica-rich organic remains (like radiolarians or diatoms) or by chemical precipitation from silica-rich fluids within limestone or chalk beds. This specimen is typically found in sedimentary strata ranging from the Precambrian to Cenozoic eras.

Uses & applications

Historically used for stone tools (arrowheads, scrapers) due to its sharp edges. Modern uses include road surfacing, jewelry (when colorful, known as jasper), and as an abrasive.

Geological facts

Chert is so hard it can scratch steel and produce sparks when struck against iron, making it a primary component for 'flint and steel' fire-starting kits. The specimen shown shows a classic conchoidal (shell-like) fracture pattern.

Field identification & locations

Identify in the field by its hardness (it will scratch glass), its waxy feel, and the way it breaks into sharp, curved edges. Commonly found in nodules or layers within limestone and dolomite formations worldwide.