gemstone

Tumbled agate or chalcedony

Microcrystalline quartz, SiO₂; likely banded chalcedony/agate

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; translucent brown, gray, and reddish zones with a smooth tumbled surface and waxy-to-glassy luster. Quartz has a trigonal crystal structure, but microcrystalline specimens show no visible crystals or cleavage; specific gravity is approximately 2.60–2.65.

Identified September 4, 2026More gemstone →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater fills cavities in volcanic rock, depositing successive layers of chalcedony and quartz. The specimen’s exact geological age and locality cannot be determined from the photograph.

Uses & applications

Used for cabochons, beads, carvings, paperweights, and small decorative objects; common tumbled pieces have modest collector value.

Geological facts

Agate’s color bands reflect changing chemistry and impurities during cavity filling; iron commonly produces red, orange, and brown tones.

Field identification & locations

Identify by its rounded polished form, translucency, waxy luster, very high hardness, and possible curved or diffuse banding; it should scratch ordinary glass but not be scratched by steel easily. Common sources include Brazil, Uruguay, India, Madagascar, Mexico, and the United States.