mineral

Onyx Agate (Brazilian Agate)

Silicon Dioxide (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Black with white/grey banding; Luster: Vitreous to waxy; Crystal structure: Trigonal (microcrystalline); Cleavage: Fractures conchoidally; Specific gravity: 2.60-2.65

Hardness: 6
Color: Black with white/grey banding
Luster: Vitreous to waxy

Identified June 9, 2026More mineral →

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Physical properties

Formation & geological history

Formed in volcanic cavities or vesicles through the deposition of silica from groundwater. The distinct banding occurs as successive layers of chalcedony fill the void over millions of years, often during the Cenozoic era.

Uses & applications

Used in jewelry, decorative ornaments, coasters, metaphysical healing practices, and precision instrumentation such as mortar and pestle sets due to its hardness.

Geological facts

Natural black agate is rare; many black 'onyx' slices on the market are actually gray Brazilian agates that have been dyed using a sugar-acid treatment to achieve the deep black color while preserving the white bands.

Field identification & locations

Identify in the field by looking for a waxy luster on fractured surfaces and rhythmic, concentric banding. Commonly found in Brazil, Uruguay, and India. Collectors value 'eye' patterns and high contrast between bands.