mineral

Banded Agate (specifically Sardonyx or Black Agate variety)

Silicon Dioxide (SiO2) - Chalcedony / Cryptocrystalline Quartz

Hardness: 6.5-7 on the Mohs scale; Color: Concentric bands of black, white, and brown; Luster: Vitreous to waxy; Crystal structure: Trigonal/Monoclinic (microscopic); Cleavage: None (conchoidal fracture); Specific gravity: 2.58-2.64

Hardness: 6
Color: Concentric bands of black, white, and brown
Luster: Vitreous to waxy

Identified March 25, 2026More mineral →

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Physical properties

Formation & geological history

Formed primarily in the cavities (vesicles) of volcanic rocks through the deposition of silica-rich groundwater. Over time, layers of silica precipitate onto the cavity walls, with impurities creating the distinct colored bands. They can range from millions to hundreds of millions of years old.

Uses & applications

Extensively used in jewelry (cabs, beads, pendants), ornamental carvings, stone seals, mortar and pestles for laboratories (due to toughness), and as decorative collectibles.

Geological facts

The word 'agate' comes from the Achates River in Sicily where they were first found. Sardonyx, the variety with brown and white bands, has been used as a protective talisman since ancient Roman times, often carved into cameos of Mars or Hercules.

Field identification & locations

Identify in the field by looking for translucent to opaque stones with parallel or concentric banding. Often found in riverbeds, gravel pits, or volcanic regions like Brazil, India, and the Lake Superior region. It is tougher than glass and will scratch a steel knife blade.

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