mineral

Polished brown agate (likely sardonyx/chalcedony)

Microcrystalline quartz, SiO₂; brown chalcedony with possible iron-oxide staining

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; dark brown, opaque to slightly translucent, with pale curved banding; waxy to vitreous luster and no visible cleavage. The smooth shape indicates tumbling or polishing.

Identified August 30, 2026More mineral →

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Physical properties

Hardness 6.5–7 Mohs; dark brown, opaque to slightly translucent, with pale curved banding; waxy to vitreous luster and no visible cleavage. The smooth shape indicates tumbling or polishing.

Formation & geological history

Forms when silica-rich groundwater fills cavities in volcanic rocks, depositing cryptocrystalline quartz in successive bands; commonly Cenozoic to older volcanic-hosted material. Exact age and locality cannot be determined from the image.

Uses & applications

Used for cabochons, beads, carvings, and decorative stones; durable but generally modestly priced as a common lapidary material.

Geological facts

Agate banding records changing chemistry during cavity filling, while brown color commonly reflects iron oxides or hydroxides. The specimen appears wet, which enhances its luster and color.

Field identification & locations

Identify by its very fine quartz texture, curved bands, glassy/waxy polish, and hardness sufficient to scratch ordinary glass. Common sources include Brazil, Uruguay, India, Madagascar, Mexico, and the United States.