mineral

Banded Agate (Onyx)

Silicon dioxide (SiO2)

Hardness: 6.5-7 on Mohs scale; Color: Black, brown, and grey bands; Luster: Vitreous to waxy; Crystal structure: Hexagonal (microcrystalline/cryptocrystalline); Cleavage: None; Specific Gravity: 2.6-2.65.

Hardness: 6
Color: Black, brown, and grey bands
Luster: Vitreous to waxy

Identified July 10, 2026More mineral →

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Physical properties

Formation & geological history

Formed in cavities of volcanic rocks or ancient lavas. Silica-rich water deposits layers of chalcedony over time, often forming parallel bands. These specimens can range from thousands to millions of years old.

Uses & applications

Used primarily for jewelry (beads, cabochons), carvings, talismans, and as polished collector pieces or pocket stones.

Geological facts

Black and brown banded agate is often commercially referred to as Sardonyx or Onyx depending on the color of the bands. Historically, it was used in Roman cameos and intaglios because the distinct layers allowed for high-contrast carving.

Field identification & locations

Identify by its characteristic parallel banding, waxy texture when polished, and high hardness (it will scratch glass). Commonly found in Brazil, India, Uruguay, and the United States.