gemstone

Polished banded agate (likely carnelian/onyx variety)

Microcrystalline quartz, SiO₂ (chalcedony with iron-oxide bands)

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; translucent brown, honey-gold, gray, and black curved banding; waxy to vitreous luster; cryptocrystalline quartz with no visible cleavage and conchoidal fracture. The smooth glossy surface indicates polishing.

Identified August 8, 2026More gemstone →

Explore Polished banded agate (likely carnelian/onyx variety) in the encyclopedia →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater fills cavities in volcanic rocks, depositing successive chalcedony layers over long periods; iron impurities create the brown and golden colors. Individual bands may form over thousands to millions of years, commonly in Cenozoic volcanic terrains.

Uses & applications

Used for cabochons, beads, carvings, ornaments, and decorative objects; durable and attractive, but this small piece is primarily a collector or lapidary specimen.

Geological facts

The concentric or curved bands are characteristic of agate, while darker translucent layers may be called sardonyx or onyx in trade. Color enhancement and dyeing are common, though this specimen appears naturally banded.

Field identification & locations

Identify by curved translucent bands, waxy polish, quartz-like hardness, and lack of cleavage; common sources include Brazil, Uruguay, Mexico, India, Madagascar, and the United States. A scratch test should be avoided on polished pieces.