mineral

Polished brown agate/chalcedony

Microcrystalline quartz (SiO₂), likely iron-stained agate or jasper

AI-generated identification · may be incorrect

Hardness about 6.5–7 Mohs; translucent brown, gray, and reddish zones with a waxy-to-glassy luster. Quartz has a trigonal crystal structure but occurs here as cryptocrystalline bands; cleavage is absent and fracture is conchoidal.

Identified September 15, 2026More mineral →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater fills cavities in volcanic rock, depositing layers of chalcedony and quartz over time; iron oxides create the brown-red colors. Most agates formed in volcanic environments during the Paleozoic–Cenozoic, though age depends on locality.

Uses & applications

Used for cabochons, beads, carvings, and lapidary collectibles; durable polished pieces are also used as ornaments and bookends.

Geological facts

The apparent dark granular patch may be included matrix or drusy quartz, while the translucent body is characteristic of chalcedony. Strong banding, if present beneath the polish, supports an agate identification.

Field identification & locations

Common in volcanic regions worldwide, including Brazil, Uruguay, Mexico, India, Madagascar, and the western United States. Field tests include conchoidal fracture, glass scratching, and lack of reaction to dilute hydrochloric acid.