mineral

Polished banded agate slab

Microcrystalline quartz (chalcedony), SiO₂, with iron-oxide and other mineral bands

AI-generated identification · may be incorrect

Hardness 6.5–7 Mohs; cream, tan, gray, and dark brown/black layers; waxy to vitreous luster; cryptocrystalline quartz with no cleavage and conchoidal fracture. The specimen appears cut and polished, possibly with a dark outer rind or contrasting matrix.

Identified September 12, 2026More mineral →

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Physical properties

Formation & geological history

Forms when silica-rich groundwater fills cavities in volcanic rock, depositing chalcedony in successive bands. Individual agates may form over thousands to millions of years, commonly in Cenozoic volcanic terrains.

Uses & applications

Used for cabochons, jewelry, bookends, decorative slabs, and lapidary work; value depends on pattern, polish, size, and locality.

Geological facts

Banded agate is a variety of chalcedony whose layers record changing chemistry and flow conditions during cavity filling. Dark bands may contain iron or manganese oxides.

Field identification & locations

Identify by its fine parallel or curved bands, glassy/waxy surface, lack of cleavage, and inability to scratch with a steel knife but ability to scratch glass. Common sources include Brazil, Uruguay, Mexico, Madagascar, and the western United States.