sedimentary

Polished dark chert or flint

Microcrystalline quartz (SiO₂), commonly called chert; possibly flint

AI-generated identification · may be incorrect

Hardness ~6.5–7 Mohs; dark gray to black with a waxy luster and smooth conchoidal fracture. Dense, very fine-grained, and non-cleavable; the glossy face appears naturally fractured or polished.

Identified August 10, 2026More sedimentary →

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Physical properties

Hardness ~6.5–7 Mohs; dark gray to black with a waxy luster and smooth conchoidal fracture. Dense, very fine-grained, and non-cleavable; the glossy face appears naturally fractured or polished.

Formation & geological history

Forms when microcrystalline quartz replaces or accumulates from silica-rich marine sediments, often in limestone. Chert ranges from Paleozoic to Cenozoic in age; the specimen’s exact age and locality cannot be determined visually.

Uses & applications

Historically used for sharp tools, arrowheads, and fire-starting; today it is mainly a collectible, lapidary, or decorative material rather than an industrial stone.

Geological facts

Its conchoidal fracture can produce extremely sharp edges. Flint is generally a dark variety of chert, especially associated with chalk or limestone formations.

Field identification & locations

Identify by testing for glass-scratching hardness, lack of visible crystals, waxy appearance, and curved fracture surfaces; it commonly occurs as nodules, river cobbles, and gravel. A close visual alternative is dark basalt, but basalt is granular and does not fracture so glassily.