Rock Identifier
Banded Agate (specifically Eye Agate / Chalcedony) (Agate (Cryptocrystalline variety of Quartz, SiO2)) — mineral
mineral

Banded Agate (specifically Eye Agate / Chalcedony)

Agate (Cryptocrystalline variety of Quartz, SiO2)

Hardness: 6.5-7 (Mohs scale); Color: Gray, brownish-black, and white bands; Luster: Vitreous to waxy; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.58-2.64.

Hardness
6
Color
Gray, brownish-black, and white bands
Luster
Vitreous to waxy
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Physical properties

Hardness: 6.5-7 (Mohs scale); Color: Gray, brownish-black, and white bands; Luster: Vitreous to waxy; Crystal structure: Trigonal (microcrystalline); Cleavage: None (conchoidal fracture); Specific gravity: 2.58-2.64.

Formation & geological history

Formed as secondary deposits in volcanic rock cavities (vesicles) or metamorphic rocks. Silica-rich groundwater precipitates layers of microcrystalline quartz over thousands of years. This specimen shows both 'eye' formations and parallel 'water-level' banding.

Uses & applications

Commonly used in jewelry (cabochons), lapidary art, decorative ornaments, and as spiritual 'worry stones'. Historically used for precision mechanical bearings due to its hardness.

Geological facts

The 'eye' patterns visible in the center are formed by rhythmic crystallization around a single point. Agates are named after the Achates River in Sicily where they were first described by Greek philosophers.

Field identification & locations

Identify by its waxy luster, translucency when held to light, and characteristic concentric or parallel banding. Commonly found in riverbeds, volcanic ash deposits, and along beaches in regions like Brazil, Botswana, and Lake Superior (USA).

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