mineral

Polished agate slice

Chalcedony (microcrystalline quartz), SiO₂, with possible druzy quartz

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; translucent gray-brown to purple-brown banded chalcedony with vitreous-to-waxy luster, conchoidal fracture, and no cleavage. The central cavity appears lined with fine quartz crystals (druzy).

Identified September 12, 2026More mineral →

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Physical properties

Formation & geological history

Agate forms when silica-rich groundwater fills vesicles in volcanic rock, depositing chalcedony and quartz in layers over time; the specimen was later cut and polished. Formation ages vary widely, commonly Paleozoic to Cenozoic.

Uses & applications

Used for decorative slices, carvings, cabochons, jewelry, and lapidary displays; agate is also a popular inexpensive collectible.

Geological facts

Its banding results from repeated changes in silica concentration, impurities, and groundwater chemistry. Brown or reddish edges commonly reflect iron-oxide staining.

Field identification & locations

Identify by concentric or irregular chalcedony banding, translucency, waxy luster, and quartz-like hardness; common sources include Brazil, Uruguay, Mexico, India, and the United States.