sedimentary

Chert

Microcrystalline Quartz (SiO2)

Hardness: 7 (Mohs scale); Color: Tan, beige, white, with orange-yellow iron staining; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 (Mohs scale)
Color: Tan, beige, white, with orange-yellow iron staining
Luster: Waxy to dull

Identified July 16, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed primarily by the accumulation of siliceous remains of marine organisms (like diatoms or radiolarians) or through the chemical precipitation of silica in groundwater within limestone or chalk beds. Period range is typically Precambrian to Cenozoic.

Uses & applications

Historically used for stone tools and arrowheads because of its sharp edges. Modernly used as road aggregate, architectural stone, and occasionally for lapidary work.

Geological facts

Chert is so hard it can scratch glass and steel. Different varieties include flint (usually dark/gray), jasper (red/brown), and agate (banded). It produces sparks when struck against steel.

Field identification & locations

Identify in the field by its hardness (scratches glass), waxy luster, and conchoidal (shell-like) fracture patterns. Frequently found as nodules in limestone regions or as river-worn pebbles.