sedimentary

Chert (Flint variety)

Microcrystalline Silica (SiO2)

Hardness: 6.5-7 (Mohs scale); Color: Cream, tan, and gray with orange-brown iron staining; Luster: Waxy to dull; Structure: Cryptocrystalline (non-visible crystals); Cleavage: None (conchoidal/shell-like fracture); Specific Gravity: 2.5-2.8.

Hardness: 6
Color: Cream, tan, and gray with orange-brown iron staining
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 radiolaria) or the chemical replacement of limestone/chalk by silica-saturated groundwater. It can occur in strata of age ranging from Precambrian to Cenozoic.

Uses & applications

Historically used for stone tools (knapping) and fire-starting. Currently used in road construction as aggregate, in ceramics, and occasionally as semi-precious tumbled stones for jewelry and collecting.

Geological facts

Chert is so hard it can scratch steel. Because of its conchoidal fracture pattern, it breaks into extremely sharp edges, which is why prehistoric humans used it for arrowheads and scrapers. The orange staining on this specimen is likely limonite or goethite.

Field identification & locations

Identify in the field by its smooth, waxy texture and curved, glass-like fracture surfaces. It will not react with acid (unlike limestone) and is often found as nodules within chalk or limestone cliffs and riverbeds.

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