sedimentary

Agate

Silicon Dioxide (Chalcedony), SiO2

Hardness of 6.5 to 7 on the Mohs scale, exhibits translucent to opaque transparency with waxy to vitreous luster. Features distinct concentric or parallel color banding. Crystal structure is microcrystalline (trigonal). No cleavage, displays conchoidal fracture. Specific gravity is 2.58 to 2.64.

Identified August 1, 2026More sedimentary →

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Physical properties

Formation & geological history

Formed primarily within the cavities of volcanic and metamorphic rocks through the deposition of silica from groundwater solutions over millions of years. Layers accumulate sequentially along cavity walls.

Uses & applications

Widely used in jewelry making, ornamental carvings, decorative slices, and historically for precision instruments like mortar and pestles due to its hardness.

Geological facts

Agate was highly prized in the ancient world as a healing stone and talisman, and the city of Idar-Oberstein in Germany became a historic center for agate cutting and polishing starting in the 15th century.

Field identification & locations

Can be identified by its distinctive banded patterns, conchoidal fracture, and waxy luster. Commonly found in volcanic regions worldwide, particularly Brazil, Uruguay, India, and parts of the United States like Oregon and Lake Superior.