sedimentary

Flint / Chert Nodules in Limestone

Cryptocrystalline Quartz (SiO2) within Carbonate Rock

AI-generated identification · may be incorrect

Hardness: 7 on the Mohs scale. Color: Tan, brown, grey, and dark blue-grey in the center. Luster: Vitreous to dull. Crystal structure: Microcrystalline/cryptocrystalline quartz (chalcedony). Cleavage: None, exhibits conchoidal fracture. Specific gravity: 2.60 to 2.65.

Hardness: 7 on the Mohs scale
Color: Tan, brown, grey, and dark blue-grey in the center
Luster: Vitreous to dull

Identified August 7, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed through the replacement of calcium carbonate in limestone by silica-rich solutions in shallow marine environments over geological timescales.

Uses & applications

Historically used for making stone tools, arrowheads, and fire-starting (striking against steel). Today used as decorative landscaping rock or in aggregate.

Geological facts

Flint and chert are fundamentally the same material, though historically 'flint' refers specifically to the dark varieties found in chalk or limestone formations. It breaks with extremely sharp edges, making it invaluable to early humans.

Field identification & locations

Can be identified in the field by its extreme hardness (scratches glass), waxy to dull luster, and distinct conchoidal fracture showing layered banding of silica replacement within a limestone host rock.