sedimentary

Polished black limestone or chert pebble

Cryptocrystalline silica (chert) or calcitic limestone; exact identification is uncertain from the photograph

AI-generated identification · may be incorrect

Small, rounded and polished specimen with a dark brown-black body, tan/cream veining, and a dull to slightly waxy luster. Chert is about Mohs 6.5–7 with no cleavage; limestone is Mohs 3 and may fizz with dilute acid. The visible veins may be calcite or lighter silica.

Identified September 8, 2026More sedimentary →

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Physical properties

Formation & geological history

Likely a naturally rounded sedimentary pebble later tumbled or polished. Chert forms from microcrystalline silica in sedimentary environments; limestone forms from compacted carbonate mud or shells, commonly during Paleozoic–Mesozoic deposition. Around 33.5°N, 94°W, gravel derived from Cretaceous and older sedimentary rocks is plausible.

Uses & applications

Used as a decorative tumbled stone, lapidary material, aggregate, and occasionally for tools if genuinely hard chert. Usually modest collector value.

Geological facts

A streak test, hardness test, and acid test would distinguish the possibilities: white streak and glass scratching suggest chert; vigorous vinegar or hydrochloric-acid fizz suggests calcite-rich limestone.

Field identification & locations

Common in river gravels and road-cut float across the south-central United States. Test an inconspicuous spot: chert scratches glass and does not fizz, while limestone is easily scratched by a steel knife and fizzes in acid.