gemstone

Polished black cabochon, likely black agate or obsidian

Chalcedony (SiO₂) if agate; volcanic glass (SiO₂) if obsidian

AI-generated identification · may be incorrect

Highly polished oval cabochon with black-to-gray coloration and possible translucent banding. Agate has Mohs hardness 6.5–7, waxy-to-glassy luster, cryptocrystalline quartz, and no cleavage; obsidian is ~5–5.5 and amorphous.

Identified August 23, 2026More gemstone →

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Physical properties

Formation & geological history

Black agate forms when silica-rich groundwater fills cavities in volcanic rock, commonly during the Cenozoic; obsidian forms when viscous lava cools rapidly, often in the Quaternary to older volcanic settings.

Uses & applications

Used in jewelry, cabochons, carvings, and ornamental objects; agate is generally more durable than obsidian for everyday wear.

Geological facts

The visible curved gray zone could indicate chalcedony banding, but the blurry image cannot distinguish agate from obsidian confidently.

Field identification & locations

Check for curved/concentric banding, translucency at the rim, and conchoidal fracture; agate scratches glass and feels less uniformly glassy, while obsidian commonly shows sharp shell-like fractures. Common sources include Brazil, Uruguay, Mexico, India, and the western United States.