sedimentary

Chert (Flint)

Microcrystalline Quartz (SiO2)

Hardness: 7 on Mohs scale; Color: Gray, black, and brownish-orange; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Hardness: 7 on Mohs scale
Color: Gray, black, and brownish-orange
Luster: Waxy to dull

Identified May 24, 2026More sedimentary →

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Physical properties

Hardness: 7 on Mohs scale; Color: Gray, black, and brownish-orange; Luster: Waxy to dull; Crystal structure: Cryptocrystalline; Cleavage: None (conchoidal fracture); Specific gravity: 2.65

Formation & geological history

Formed primarily as a chemical precipitate from silica-rich fluids or through the accumulation of siliceous skeletal remains (like radiolaria) in deep-sea environments. This often occurs as nodules within limestone or chalk beds.

Uses & applications

Historically used for making stone tools (arrowheads, scrapers) and fire-starting (flint and steel). Modern uses include road surfacing, decorative landscaping, and as an ingredient in ceramics.

Geological facts

Chert is so hard it can easily scratch glass. When struck against steel, it produces sparks, which led to its extensive use in flintlock firearms and for starting fires throughout human history.

Field identification & locations

Field identification focuses on its waxy luster, extreme hardness, and distinctive conchoidal (shell-like) fracture patterns. It is commonly found in sedimentary basins and as water-worn pebbles in stream beds.