mineral

Polished black agate slice

Chalcedony (microcrystalline quartz), SiO₂, with iron/manganese staining

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; dark gray-black concentric banding, waxy to vitreous luster, cryptocrystalline structure, no cleavage, specific gravity about 2.6. The translucent brown-gray rim and curved bands are characteristic of agate.

Identified August 9, 2026More mineral →

Explore Polished black agate slice in the encyclopedia →

Identify your own rocks.

Get a report just like this from any photo, free.

App Store Google Play

Physical properties

Formation & geological history

Forms as layered chalcedony filling cavities in volcanic rocks, commonly during silica-rich groundwater circulation. Individual deposits range from Cenozoic to much older, depending on the host volcanic rock.

Uses & applications

Used for cabochons, jewelry, bookends, ornaments, and lapidary slices; polished specimens are popular collectibles. This appears to be a cut and polished, possibly dyed, specimen.

Geological facts

Agate banding records episodic deposition of silica layers, often following the cavity walls. Black coloration may be natural from manganese/iron oxides or enhanced by dye.

Field identification & locations

Identify by concentric curved bands, translucent edges, waxy luster, and inability to scratch it with a steel knife; quartz should scratch glass. Common sources include Brazil, Uruguay, Mexico, India, Madagascar, and the United States.