sedimentary

Chert

Microcrystalline quartz (SiO2)

Hardness: 7 on the Mohs scale. Color: Brown, grey, white, yellowish, or reddish-brown. Luster: Waxy to vitreous. Crystal structure: Microcrystalline/cryptocrystalline (chalcedony/quartz). Cleavage: None (conchoidal fracture). Specific gravity: 2.60-2.65.

Hardness: 7 on the Mohs scale
Color: Brown, grey, white, yellowish, or reddish-brown
Luster: Waxy to vitreous

Identified July 28, 2026More sedimentary →

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Physical properties

Formation & geological history

Forms by the chemical precipitation of silica from aqueous solutions, or via the replacement of calcium carbonate in limestone by silica-rich fluids during diagenesis. Found globally in sedimentary sequences, particularly within chalk and limestone formations.

Uses & applications

Historically used for prehistoric stone tools (arrowheads, blades, scrapers) due to its sharp conchoidal fracture. Today used locally in road aggregate, abrasives, and sometimes decorative landscaping.

Geological facts

Chert that is dark or black in color is often referred to as flint. Because of its durability and conchoidal fracture, it was one of the most vital materials for early human toolmaking.

Field identification & locations

Identify in the field by its extreme hardness (scratches glass), waxy luster, and characteristic conchoidal fracture with razor-sharp edges. Commonly found as nodules or layers embedded in limestone cliffs and riverbeds.