mineral

Polished agate slice

Agate, a banded variety of chalcedony (microcrystalline SiO₂)

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; translucent gray, cream, and brown concentric banding; waxy to vitreous luster; cryptocrystalline quartz with no visible cleavage. The irregular outer rind appears natural and may retain druzy quartz or host-rock material.

Identified August 12, 2026More mineral →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater fills cavities in volcanic rocks, depositing successive layers of chalcedony over millions of years, commonly during the Cenozoic or older volcanic episodes. This specimen has been cut and polished.

Uses & applications

Used for cabochons, jewelry, bookends, decorative slabs, and lapidary ornaments; attractive slices typically sell for about $15–$75, with larger or unusually patterned pieces reaching $100–$300.

Geological facts

The pale concentric bands record changing chemistry and fluid conditions during cavity filling. Agate is a variety of quartz rather than a separate mineral species.

Field identification & locations

Identify by curved or concentric banding, translucency, quartz-like hardness, and inability to scratch with a steel knife; it should scratch glass. Common sources include Brazil, Uruguay, Mexico, India, Madagascar, and the western United States.