mineral

Tumbled agate or chalcedony nodule

Microcrystalline quartz (SiO₂), likely agate/chalcedony with iron staining and possible druzy quartz

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; translucent gray-brown to honey-yellow with a waxy-to-glassy luster and a small crystalline cavity. Quartz has a trigonal crystal structure, typically no visible cleavage, and a specific gravity near 2.6; the rounded shape indicates tumbling or natural abrasion.

Identified September 18, 2026More mineral →

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Physical properties

Formation & geological history

Silica-rich groundwater deposited layered chalcedony in a cavity of volcanic rock, commonly basalt; later weathering liberated the nodule. Agates formed mainly during the Cenozoic and late Mesozoic, depending on locality.

Uses & applications

Used as a lapidary, cabochon, ornamental, and collecting material; quality is generally decorative rather than gem-grade.

Geological facts

The visible cavity may contain quartz druse or iron-oxide inclusions, producing the dark metallic-looking patch. A polished or tumbled surface can obscure characteristic banding.

Field identification & locations

Look for curved translucence, waxy luster, conchoidal fracture, and possible concentric bands under strong light; verify with a scratch test only on an inconspicuous area. Common sources include Brazil, Uruguay, Mexico, Madagascar, and the western United States.