gemstone

Polished banded agate (likely sardonyx/onyx agate)

Microcrystalline quartz, SiO₂ (chalcedony with iron-oxide and possibly manganese-rich bands)

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; gray-black, tan, and reddish-brown parallel bands; waxy to vitreous luster; cryptocrystalline quartz with no visible cleavage. The smooth, rounded form appears polished or tumbled, possibly two pieces stacked together.

Identified September 3, 2026More gemstone →

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

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Physical properties

Formation & geological history

Forms when silica-rich groundwater fills cavities in volcanic rocks, depositing chalcedony and colored impurities in successive layers. Individual bands may form over thousands to millions of years; exact age and locality cannot be determined from the image.

Uses & applications

Used for cabochons, beads, carvings, bookends, and ornamental objects; durable enough for some decorative and lapidary applications.

Geological facts

Agate’s banding records changing chemistry during cavity filling, while iron oxides commonly produce red, orange, and brown tones. Dark bands may be natural or enhanced by polishing and treatment.

Field identification & locations

Identify by its translucency at thin edges, curved concentric or parallel banding, waxy luster, and resistance to scratching by a steel knife; it should not fizz in dilute acid. Common sources include Brazil, Uruguay, India, Mexico, Madagascar, and the United States.