mineral

Tumbled black-and-white banded stone, likely onyx or agate

Cryptocrystalline quartz (SiO₂), probably banded chalcedony/onyx

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; opaque black with a contrasting white band, smooth tumbled surfaces, and dull to waxy luster. Quartz is microcrystalline and lacks visible cleavage; the specimen appears water-worn or polished.

Identified August 8, 2026More mineral →

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Physical properties

Formation & geological history

Formed when silica-rich groundwater deposited successive layers of chalcedony in a cavity or fracture, producing the banding. Such deposits are generally Cenozoic to older, depending on locality.

Uses & applications

Used as an ornamental stone, cabochon, carving material, and inexpensive jewelry or collectible; not typically used structurally.

Geological facts

The term “onyx” is commonly applied to parallel-banded chalcedony, while irregularly curved bands are more accurately called agate. The black color may be natural or enhanced by treatment.

Field identification & locations

Identify by its very hard, glass-scratching surface, waxy luster, and sharply contrasting silica bands; it should not fizz in dilute acid. Common sources include Brazil, Uruguay, India, Mexico, and the United States.