mineral

Banded Agate (specifically Black Sardonyx/Onyx variety)

Silicon Dioxide (SiO2) - Cryptocrystalline Quartz

AI-generated identification · may be incorrect

Hardness: 6.5-7 on the Mohs scale. Color: Alternating bands of black, grey, and white. Luster: Vitreous to waxy. Crystal Structure: Trigonal (microcrystalline). Cleavage: None. Specific Gravity: 2.60-2.65.

Hardness: 6
Color: Alternating bands of black, grey, and white
Luster: Vitreous to waxy

Identified April 4, 2026More mineral →

Explore Banded Agate (specifically Black Sardonyx/Onyx variety) in the encyclopedia →

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Physical properties

Formation & geological history

Formed through the deposition of silica from groundwater in the cavities of igneous rocks, particularly volcanic lavas. The banding is created by rhythmic crystallization over thousands of years.

Uses & applications

Primarily used for ornamental purposes, jewelry (pendants and rings), sculptures, bookends, and collection specimens. Historically used for making cameos and intaglios.

Geological facts

This specimen features a druzy quartz center, where larger crystals grew into the open pocket of the geode. Agates were named by the Greek philosopher Theophrastus who found them along the Achates River in Sicily.

Field identification & locations

Identify by its characteristic parallel, concentric banding and waxy texture. In the field, look for nodules with a rough, weathered exterior but translucent/banded interior when cracked or cut. Commonly found in Brazil, India, and Germany.